Brain: The organ with which one thinks and feels.
This definition makes a lot of practical sense, since the brain is an organ - indeed the best protected and isolated organ that one has - and the fact that it is necessary for one's thinking and feeling has been experimentally demonstrated in many ways.
Consequently, the thesis that thinking and feeling are activities of a living brain is quite plausible. It is not certain, and one problem for the thesis is that so far there is remarkably little knowledge about how the brain produces its marvels.
The opposite thesis, that one's thinking and feeling in fact is done by something called a soul, that is independent of the brain, even if somehow associated with one's brain and body as long as one is alive, and that may survive one's death, seems far less probable, at least as long as there are no souls evidently roaming about as there evidently are living bodies roaming about.
Incidentally, a personally convincing example of how dependent one's feelings are on physical facts is local anaesthesia, like dentists may supply: What hurt very much before, is totally painless with a local anaesthetic.
In any case: A well-supported scientific hypothesis about what one - one's self - really is, that is far better supported than religious hypotheses that one is an immortal soul only very briefly associated with one's present body, is that one is part of the working of one's living brain: one's self on this hypothesis is a set of processes happening inone's brain
Apart from such religious misgivings as one may have about this hypothesis - see Crick - the main problem with it is that so far there is much that is not understood about the brain, which is not so amazing, since it has an extra-ordinarily complex organ that has been seriously studied in a scientific way only for a few generations.