Some people define some things (which they truly may be or are) Impossible. 'Impossible' has a humane meaning in itself. But... If 'something' is really impossible... then why can you think that? If something is impossible... then why did the neurons in your brain have that thought? It must've been impossible for them to think of something which is not possible.

I'll assume, just for simplicity, that by a "thought" you mean a belief and by "something impossible" you mean a proposition that cannot possibly be true. I hope my assumptions aren't off the mark. (I'm not a neuroscientist, so I'll say nothing about how neurons work.)

If my assumptions are correct, then your question becomes "How can anyone believe a proposition that cannot possibly be true?" One answer is this: "Easy! For example, many people down through the ages believed that they had accomplished the famous geometric construction known as squaring the circle. But the proposition they believed cannot possibly be true, because squaring the circle is impossible, as was finally proven in 1882. Those who believed the proposition obviously didn't see the impossibility of the construction."

An opposing answer is this: "They can't! Indeed, we can understand the behavior of those misguided geometers only if we attribute to them a false belief that could have been true, such as the belief that a particular mathematical sentence expresses a true proposition. The mathematical sentence in fact doesn't express a true proposition, but it would have had it expressed an appropriately different proposition, which it could have." For more on this answer, google the article "Rationality and Believing the Impossible," by Ruth Barcan Marcus.

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