I agree with Hume that extreme scepticism about the outside world isn’t practical. ARE there any good arguments against solipsism when not being practical?
Answer by Helier Robinson
In principle there are two ways to refute solipsism: show it to be self-contradictory, and prove that something outside your present consciousness (outside the content of solipsism) actually exists. My own view is that the first does not work because solipsism is consistent, but the other can be done with the following argument. The concept of ‘intrinsic necessary existence’ is not self-contradictory, so it must exist in at least one possible world; in that world it exists necessarily, and so that world exists necessarily (if a part of a whole exists then the whole exists); therefore at least one possible world exists necessarily, and so something exists outside your present consciousness. This necessary world is, of course, the world we live in.
Answer by Geoffrey Klempner
Whether or not solipsism can disproved by showing that it is self-contradictory or (as Helier claims) by showing that at least one thing exists outside your present consciousness, the question remains whether solipsism is a practical possibility. What is the difference between extreme scepticism about an external world and solipsism? If nothing exists outside my present consciousness, then a fortiori (from the stronger premiss) no other conscious beings exist apart from my present consciousness. So I would be interested in a version of solipsism which grants the existence of an external world, but denies that other consciousnesses exist, or are ‘real’. This would be equivalent to ‘extreme scepticism about other minds’.
This would be the belief demonstrated in action by an extreme psychopath who regards other persons or conscious beings as merely tools to use or obstacles to overcome. Whether such individuals are ever found in the ‘pure’ form is an empirical question. But there seems to be no reason why, in principle, a ‘practical solipsist’ could not live and function successfully – on ‘its’ own terms.