Thank you for your question. For a long while in the history of philosophy it was thought that what was conceivable was a good indication of what was possible. Descartes is a good example of this way of thinking, though he was careful to require that not any old conceiving of a thing showed it to be possible. Rather he required that the conceiving had to be "clear and distinct", meaning roughly that it had to pass the most stringent standards we can muster to make sure the conceiving is coherent (i.e., not subtly self-contradictory). In the middle of the 20th century this methodology began to break down. For instance, in the Sixties Hilary Putnam distinguished between concepts and properties, making clear that our concepts of things like gold may not reveal its true properties. Similarly, Kripke's notion a decade later of "natural kinds" made room for the possibility that what is "metaphysically possible" may not correspond to that is conceivable.

This issue is still a topic of intense debate. Some philosophers in the last decade or so have argued that conceivability considerations have *some* force in determining what is genuinely possible. For instance, see Frank Jackson's, From Metaphysics to Ethics: A Defense of Conceptual Analysis, Oxford University Press, 1997. I should mention that conceivability does seem an important tool for many fields, not just philosophy: for instance, physics uses thought experiments regularly. Those "experiments" are constrained by what we know of the laws of physics, but philosophers' thought experiments can take into account all we know also about the empirical world. Imagine, further, how hard it would be to reason in ethics or political philosophy without being about to construct thought experiments! The moral here is that conceivability considerations are not aimed at finding out about our minds, but rather are our attempts to use common sense, albeit fallibly, to find out about the world.

Mitch Green