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Analogies, Slippery Slopes & the Prohibition of Cannabis

Robert Davies applies some critical thinking to an old debate.

The aim of this article is twofold. One of its purposes is to try and give some reasons why the laws that prohibit the use of cannabis are unjustified. To achieve this I will examine two common arguments put forward to justify why cannabis is illegal and explain why I think they ought to be rejected. Its second aim is not really to do with the prohibition or legalisation of cannabis at all; rather I want to show how and why two different types of philosophical arguments fail. In doing so I shall demonstrate the way in which a philosopher goes about analysing and evaluating arguments which take a particular form. The two forms of arguments I shall consider are arguments by analogy and slippery slope arguments.

Arguments by Analogy

Analogies rest upon shared properties. An argument by analogy attempts to show that because two things share a certain property, and because a certain additional property is true of one of these things, it is also true of the other. Take the argument below:

Rob is like Bill
Bill likes Pulp Fiction

Therefore, Rob likes Pulp Fiction

In this example the common property is that Rob ‘is like’ Bill. In order for us to accept the argument by analogy there need to be similarities between Rob and Bill that are relevant to the assertion that Rob also likes Pulp Fiction. If, for instance, what Bill and Rob have in common is that they like the same sort of films then the argument by analogy seems a good one. Note, however, that the argument is still not formally valid. The standard definition of validity is that it is not possible for the premises to be true and the conclusion false. Even if we know Rob and Bill like the same sort of films, it is possible that Robert does not like all the films that Bill likes, and Pulp Fiction might be one of these films. To make the argument formally valid requires a premise such as “Robert and Bill like all the same films”. This type of argument is valid although strictly speaking it is not an argument by analogy. As it stands our argument by analogy is formally invalid but it is nonetheless convincing. However, if the similarities between Bill and Rob are of a different kind we might find the argument unconvincing. If the similarity between Rob and Bill is that they look like one another this does not provide good grounds for asserting that they like the same films. The analogy breaks down because the additional property of liking Pulp Fiction is not relevant to the common property of looking like one another.

The argument for the prohibition of cannabis is often founded upon analogy. This time the analogy concerns the properties of drugs rather than the properties of people. The argument looks like this:

Cannabis is like heroin
Heroin is a dangerous drug

Therefore, cannabis is a dangerous drug

Before looking at the analogical part of the argument it is worth noting two features of this argument for prohibiting cannabis. First, we shall take the second premise above to be true because heroin possesses a number of properties generally thought to be hazardous. Second, to get from the first conclusion that cannabis is dangerous to the conclusion that cannabis ought to be illegal requires a second argument such as the one set out below:

Cannabis is a dangerous drug
Dangerous drugs ought to be illegal

Therefore, cannabis ought to be illegal.

Again, I think we ought to accept the truth of this second premise in this argument. Only the most trenchant of libertarians will be inclined to permit such dangerous activities as the use of heroin. The second part of the argument is formally valid. If we accept both that cannabis is dangerous and that dangerous drugs ought to be illegal, it follows that cannabis ought to be illegal. It is not possible for the premises of this argument to be true and the conclusion false.

If we accept the truth of the second premise of the second argument, that dangerous drugs ought to be illegal, and given that the argument is a valid one, then whether or not the second argument is sound depends upon the truth of its first premise. Although it is not possible for the premises to be true and the conclusion false, the argument might be rejected as unsound because one of the premises is false. An argument is sound if, and only if, it is valid and it has true premises. In this context, whether we regard the first premise in the second argument as true, in turn, depends upon whether or not we accept the analogy between cannabis and dangerous drugs in the first argument. So our assessment of the analogy between cannabis and dangerous drugs such as heroin will determine whether we accept the whole argument for the prohibition of cannabis. Ought we to accept the analogy between cannabis and heroin?

Clearly cannabis and heroin share some properties; they are both drugs and are both on the whole used recreationally. But these two properties are not relevant to us attributing the additional property of being dangerous to cannabis, unless we take the rather pious view that if something is used for recreation then it is dangerous. Another reason we might think a drug to be dangerous is because it can lead to overdoses. Many illegal drugs can lead to overdoses and it seems a good reason to stop people from taking them. Heroin, cocaine, amphetamines and barbiturates can all kill people if taken in excess and so ought to be thought of as dangerous. Experiments on rats, however, show that it takes approximately seven times their body weight in cannabis to kill them. It follows that if this is true of humans, then there is little likelihood that anyone will ever manage to smoke or swallow enough cannabis to overdose. The analogy between cannabis and dangerous drugs like heroin fails to hold, since cannabis cannot lead to overdoses.

A second reason which might be considered relevant to classifying a drug as dangerous is its potential for addiction. Addiction seems quite clearly to be a bad thing. Philosophers’ concerns about addiction often stem from the restriction it places upon an individual’s freedom, or their autonomy. Autonomy is a person’s capacity to make free choices. Animals are not thought to be autonomous as their choices are determined by their instincts. Humans, on the other hand, possess the capacity to make their own choices. To speak figuratively, in the case of an addict, it is the drug rather than the individual which determines the choices he or she makes. The addict whose autonomy is sacrificed to a drug is no more free than an animal acting on instinct. However, establishing whether or not a drug is actually addictive is problematic. The tricky notion of psychological dependence is often dispensed with because it is difficult (if not impossible) to verify whether someone is in fact psychologically dependent. Instead physical dependence is put forward as a measure of whether a drug is addictive. If a drug is physically addictive then withdrawal from the drug causes physical symptoms. It is said that the threat of the physical symptoms of withdrawal keeps users addicted to a particular drug. According to this measure of addiction, many of the drugs mentioned above are addictive, such as cocaine, heroin and amphetamines, yet there is no evidence to suggest that cannabis is physically addictive. The analogy between the properties of dangerous drugs and the properties of cannabis once more appears unsustainable.

A third reason which might be relevant to assessing the danger of a drug is the drug’s capacity to lead to serious or life-threatening medical conditions. Heroin use can lead to a variety of medical problems ranging from disruption of eating habits, through damage to blood vessels and clogged lungs, to potentially fatal diseases such as Hepatitis C and AIDS. The continued use of other drugs can also lead to serious medical conditions; the prolonged use of LSD and other hallucinogenics has been linked to severe psychological disorders, such as schizophrenia. Cannabis use, however, has not been firmly linked to any potentially fatal or serious medical conditions. Research has only established that cannabis can produce some short-term memory loss. Once again the analogy between cannabis and dangerous drugs breaks down.

One final reason for thinking that a drug is dangerous is that it produces violent behaviour in the user. This is true of certain stimulants such as PCP, but cannabis is a sedative which seems to preclude violent or erratic behaviour. It looks, then, as if we ought to reject the analogy between cannabis and dangerous drugs such as heroin altogether. There appear to be insufficient grounds to assert that cannabis shares with heroin the additional property of being dangerous. Therefore, the first premise of the second argument ought to be rejected, and consequently the conclusion that cannabis ought to be illegal, since it is only dangerous drugs that ought to be prohibited.

Perhaps one way to save the argument is to change our definition of what constitutes a dangerous drug. We said that cannabis has some side-effects, such as loss of memory. It might be argued that these mild effects are sufficient to classify a drug as dangerous. If these effects are sufficient, and dangerous drugs ought to be illegal, then we can accept the cannabis is dangerous and conclude that cannabis ought to be illegal. That is, a drug possessing (at least) mild side-effects is a relevant reason to hold that cannabis and heroin both share the property of being dangerous. However, if we accept this line of reasoning then we must also accept that many other drugs including alcohol and nicotine are also dangerous and so ought to be illegal. If cannabis is included in our definition of dangerous drugs then so too must alcohol and nicotine, since they are at least as dangerous as cannabis. In fact, alcohol and nicotine are more dangerous; they can both cause serious medical problems, are both addictive, and alcohol can produce overdoses as well as violent behaviour in some drinkers. So if cannabis is to be prohibited, then other potentially dangerous legal drugs, such as alcohol and nicotine, ought also to be banned.

To summarise, the analogy between cannabis and dangerous drugs like heroin, upon which our argument for prohibition depends, appears to be unsustainable. Cannabis shares some properties with illegal drugs but it does not share the additional property of being dangerous. There seem to be no relevant reasons to think that cannabis and heroin share the additional property of being dangerous. It follows that the key premise upon which the argument for prohibition rests ought to be rejected, namely that cannabis is dangerous. If, on the other hand, the definition of dangerous is widened so that cannabis is considered dangerous, then we must also accept an analogy between drugs like alcohol and nicotine and drugs like heroin, since alcohol and nicotine are at least as dangerous as cannabis. In this case if the weakened analogy provides grounds for prohibiting the use of cannabis then we ought (for consistency) to accept that alcohol and nicotine also be prohibited. But this conclusion is one we are unlikely to endorse, so again it seems we are forced back into rejecting the analogy. And of course rejecting the original analogy means that the argument for the prohibition is unconvincing. So what other argument might we use to justify the prohibition of cannabis?

Slippery Slope Arguments

As the name suggests, a slippery slope argument is an argument in which one premise leads towards another down a ‘slippery slope’. Usually the slippery slope ends in a premise that one clearly should reject. It is then asserted that the original premise ought also to be rejected in order to prevent the descent down the slope. Again, let us start with an example:

If Rob has one drink then he will have another
If Rob has two drinks then he will be sick
Rob does not want to be sick

Therefore, he should not have a drink

This argument is also formally invalid. If we accept all the premises we can still reject the conclusion. Although Rob might not want to be sick, his need for a drink might be so pressing that he is prepared to take the risk. Whether or not the argument is convincing depends upon whether or not each of the conditional premises which leads down the slippery slope is accepted. Namely, that one drink leads to another and so on until Rob is sick. This is going to depend upon whether there are good grounds to suppose that Rob having one drink will cause him to have another and so on. If Rob is an alcoholic with a stomach ulcer then there seem to be good grounds for thinking the slippery slope convincing. On the other hand, if Rob is a moderate drinker with an iron stomach then it is reasonable to reject the conditional premises that lead down the slippery slope.

We can set out the slippery slope argument for the prohibition of drugs in a similar way.

If a person takes cannabis it will lead them to take other drugs
These other drugs ought to be illegal

Therefore, cannabis ought to be illegal

It seems that the second premise is solid enough. If we take ‘other drugs’ to refer to drugs such as heroin and cocaine our previous discussion indicated some good reasons why these drugs ought to be illegal. Thus, whether or not we accept this argument as convincing will depend upon whether or not we accept the conditional premise that leads down the slippery slope, namely that if a person takes cannabis it will lead them to take other illegal drugs. Is it the case that taking cannabis will cause a person to take other drugs?

It is known that far more people take cannabis than more dangerous drugs. Surveys indicate that approximately one third of people between 11 and 16 years old have tried cannabis, whereas the numbers of the whole population who have used heroin is below 3%. Therefore, roughly ten times as many people use cannabis as use heroin. It follows that even if all those people who take dangerous drugs began by taking cannabis there would still be far more people who just take cannabis and do not go on to take harder drugs. It is not even probable that taking cannabis will lead to other drugs, it is far more likely that it will not. Another consideration further breaks down the causal slippery slope, namely the very fact that cannabis is illegal. The very illegality of cannabis is partly responsible for strengthening the causal connection between soft drugs and hard drugs.

How often when you go shopping do you end up buying something that you do not really want? I often go to the supermarket for one thing and come back with another. I have not, however, gone to a chemist and returned with a bottle of wine. This is because these two products are purchased from different sources. It is also the case with drugs. Legal drugs are bought from the chemist or the off license, whereas illegal drugs are purchased on the black market. It is likely that the same drug dealer who sells cannabis will also be able to supply, or have access to the supply of, more dangerous drugs. The very fact that cannabis is illegal seems, perversely, to make it more likely that we will lead people to take other drugs because they are available from the same source. If the aim of drugs policies are to stop people taking dangerous drugs it would be more sensible to make cannabis legal, because it would be less likely that taking cannabis would cause a person to take other drugs, since they would have to go somewhere else to make their purchase.

The slippery slope argument also fails to provide grounds for prohibiting the use of cannabis. The evidence suggests that it is unlikely that cannabis use will cause a person to take other drugs. What is more, when cannabis use does cause users to move on to harder drugs then the reason is often because cannabis, like hard drugs, is illegal.


If we return to the two aims set out at the beginning of this article, how have we fared? As for the first objective, the two arguments under consideration both failed to justify the prohibition of cannabis. However, just because those two arguments fail does not mean that there can be no good arguments for prohibiting the use of cannabis. All we have shown is that these arguments fail, not that all arguments for prohibiting cannabis must fail. I suspect, however, that there are few arguments for prohibiting cannabis that are immune to the points raised here. In particular, the inconsistency between a dangerous drug like alcohol being legal, whilst the far less dangerous use of cannabis is prohibited, is going to make it very difficult to provide a watertight reason as to why cannabis should be prohibited when alcohol is not. The reason for the inconsistency probably has far more to do with social and cultural reasons than with philosophical arguments. Alcohol has been around in our society much longer than cannabis. Things that are new are often thought of as dangerous and perhaps this is not such a bad thing. It is often sensible to err on the side of caution. Our discussion, however, highlights the fact that the dangers of cannabis are more perceived than real, so this caution is unnecessary.

My second aim was to show how certain types of philosophical arguments fail. Both the argument by analogy and the slippery slope argument are formally invalid. However, an argument can still be convincing although it is formally invalid. An argument by analogy is going to be convincing if we can find a relevant reason to attribute the additional property that one thing holds to the other thing. In the first part of this article it was shown that there were no relevant reasons to think that cannabis and heroin shared the additional property of being dangerous. The only way that we could attribute the additional property of being dangerous to cannabis is if we argue that producing mild side effects was sufficient to make a drug dangerous. However, then we are compelled to endorse the unacceptable conclusion that lots of other drugs which are legal are also dangerous and ought to be prohibited. Slippery slope arguments are convincing if the conditional premises are accepted, that is that one event causes another. In the second section it was shown that cannabis use rarely leads a user to take other drugs and when it does this might be because cannabis is illegal.

The examination of the two different forms of argument illustrates how the process of setting out and examining certain forms of arguments helps in assessing whether or not they are good arguments. If we can analyse the form of an argument it is going to be much easier for us to critically evaluate it, since we know at what points an argument is going to be susceptible to criticism. To sustain an analogy we need to know that the additional property is relevant to the shared properties and if it is not then we ought to reject the analogy; and to sustain a slippery slope argument we need to be convinced that one event in the sequence will cause the next. In everyday life we put forward and reject arguments of the kind we have encountered in this article, but the ability to analyse these arguments is rarely taught or learnt. Without this skill we are liable to accept arguments that we ought to reject.

By examining the logical form of some of the arguments put forward to justify the prohibition of cannabis, I have tried to show that we ought to reject them. Perhaps the fact that many people find these arguments convincing illustrates that they lack the critical skills necessary to analyse and evaluate arguments generally. If they had such skills then perhaps other unfounded arguments and beliefs that are widely accepted might also be called into doubt.

© Dr Robert Davies 2005

Robert Davies has a PhD in Philosophy from the University of Leeds.

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