hello. what is the field/or term in philosophy that describes or investigates What or What isn't possible in this existence? (what is possible/impossible).
if concepts are of the mind and not of physical reality, does it exist? (For example, unicorns, extraterrestrials, ghosts, redness).
There are several areas of philosophy that deal with the questions you raise:
1. Metaphysics, which is the study of reality, i.e., the branch of philosophy that attempts to answer the question: what is real.
2. Epistemology: is the branch of philosophy that deals with the nature of knowledge.
3. Logic: is the science of evaluating arguments and the search for the perfection of proof. Logic is the tool that is used by metaphysics and epistemology to intelligently discuss the nature of truth, knowledge, and belief.
Regarding the nature of concepts and physical reality, there many answers, each depending upon the metaphysical presumptions made. Idealists tend toward prioritizing the nature of the idea and would answer that concepts are more real than physical objects.
Materialists/Empiricists would argue the converse, that physical reality is all there is and concepts and ideas are only epiphenomena that cannot come into being without some physical brain or sentient being thinking them into existence.