I asked a similar question a few years ago, but I have never found an answer. In philosophy, people often debate about whether an entity or state of affairs is conceivable. One example is philosophical zombies, but there are others, such as unicorns or fairies. These debates seem to go nowhere, and I think it is because there is no good formal definition of conceivable. So, my question is, has anyone ever given a rigorous and formal definition (or at least a clarification of the term) of conceivable, and if so, can someone direct me to such a book or paper? I recently asked what the definition of intension was, and I got a satisfactory response when I saw that other people have been thinking about these things. I am sure I am not the first one to wonder about a definition of conceivable.