The Philosophical Issue Regarding Knowledge
The largest philosophical issue regarding knowledge is the casting of knowledge in a philosophical form that reveals its reality and essence and shows whether it is a material phenomenon present in matter when matter reaches a certain stage of development and completion, as materialism claims, or a phenomenon free from matter and, together with its manifestations, supported by a certain kind of existence, as it is understood philosophically in metaphysics.
Since Marxism is a materialistic school, it of course emphasizes the materialistic notion of thought and knowledge. This is made clear in the following texts from Marx, Engels, Georges Politzer and Roger Garaudy, respectively:
Regardless of the apparent superiority of our consciousness and thought, they are nothing but a bodily or material organic product - this being the brain. 
Thought is a historical product of nature's development to a high degree of perfection represented in the sense organs and nervous system of the living species, especially in the highest central part which rules the whole organic being, i.e. in the brain. 
The philosophical notion of knowledge is not the only notion of knowledge (p. 373) worthy of research and study, for knowledge is the meeting point of many [types of] research and studies. Every scientific discipline has its own notion that treats one of the many problems concerning knowledge, and one aspect of the secrets of the intellectual life whose mysteriousness and complexity make it exciting. Behind all these scientific notions lies the philosophical notion in which conflict between materialism and metaphysics arises, as mentioned earlier. The present issue, therefore, is the subject of different types of philosophical and scientific discussions.
On the basis of this error, the materialistic claim was established, this is the claim which asserts that knowledge in the philosophical notion of metaphysics is incompatible with knowledge in the scientific notions.
We have already seen how Georges Politzer attempted to prove the materiality of knowledge from a philosophical point of view by means of pieces of evidence drawn from the natural sciences. Others also made the same attempt.
We have already remarked that the aspects of knowledge touched upon or treated by those scientific studies are many, owing to the relation of the sciences to the various aspects of knowledge, rather due to the fact that a science has a variety of scientific schools, every one of which investigates knowledge from its own specific point of view. Physical and chemical researches, for example, explore certain aspects of knowledge.
Physiology has its own share in exploring knowledge; also psychology, with its various schools, including the schools of introspectionism (al-istibtaniyya),  behaviorism, functionalism (al-wazifiyya),  and so on.
Every one of these schools studies a various aspect of knowledge. After all of this, the role of philosophical psychology emerges to treat knowledge from its own perspective. It investigates whether knowledge in essence is a material state of the nervous system or a pure spiritual state.