Epistemology is the branch of philosophy concerned with knowledge. Epistemologists study the nature, origin, and scope of knowledge, epistemic justification, the rationality of belief, and various related issues. Epistemology is considered one of the four main branches of philosophy, along with ethics, logic, and metaphysics. The branch primarily deals with the philosophical analysis of the nature of knowledge and the conditions required for a belief to constitute knowledge, such as truth and justification.
After Socrates - when the basic pillars of philosophy were already invented - only epistemology was missing. That one can write about it endlessly by transferring "thinking" to a meta-level is communicated in the meme by the fact that we will need many many scrolls more to treat this topic properly. With this, the competences and fields of activity of a philosopher were also largely marked out: ethics, aesthetics, metaphysics (cosmogony), anthropology (first as psychology) natural philosophy as a precursor of the modern and later to be parceled out natural sciences – and finally epistemology.