Some believe that it is reasonable for people to do what they want provided they don’t harm anyone. Yet the statement itself has several flaws that make this idea problematic. The problems lie in the assumptions the statement is based on as well as definitional difficulties.
The most obvious argument against the statement that people should be able to do whatever they want provided they don’t harm anyone is that our actions always affect others. Given that humans are social beings we are always involved in other people’s lives as they are in ours. So while the argument may, for example, support the idea that people can take illicit drugs since it is their body they are choosing to affect, their behavior and personality may change in such a way as to hurt others. If they are parents they may not take care of their children adequately or they may inadvertently model drug seeking and taking behavior. The children may then grow up to become addicted to drugs because of what they learned in their family.

Similarly, as we may influence others to engage in problematic behavior they may influence us equally. We behave as we do largely because we were taught this through the antecedents and consequences of what we do. No one decides to act a certain way in a vacuum without anyone else ever influencing their decisions. Whether we react to something in accordance with it or against it, there is always someone who has influenced our view of the world and how we choose to act in response to this view. So while someone may behave in a certain way that make them suffer, it is difficult to argue they came up with the behavior without it having been influenced by at least one other person. The second person will likely go on to influence others whether they intend to or not.

Finally, there is the matter of who defines what it is to harm someone? If someone engages in a particular behavior with the belief they aren’t harming anyone to ensure this would require defining harm exactly the same as everyone else who they may influence knowingly or unknowingly at the time as well as in the future. Since everyone defines harm slightly differently this means that while a person may not be hurting someone based on their own definition of harm they may be hurting someone based on that person’s definition. Additionally, while someone may not be harmed in the present, it impossible to know whether the behavior can have some effect on someone in the future. These arguments show why the argument that a person should be allowed to do what they want as long as they don’t harm others is not just unreasonable but virtually impossible to accomplish.