What is the term for the thick, viscous substance that serves as the physical basis of all living things?

Protoplasm is indeed the term that refers to the thick, viscous substance found within living cells and serves as the physical basis of all living organisms. It is the jelly-like fluid that contains various cell organelles, such as the nucleus, mitochondria, and ribosomes, all of which are essential for the cell's functions and overall life processes. Protoplasm is critical as it provides the medium where metabolic processes occur, allowing cells to grow, reproduce, and respond to their environment.

The other terms listed do not refer to this concept of living material. Psychiatry pertains to the diagnosis and treatment of mental health disorders. Proximity relates to nearness in space, time, or relationship, which is not relevant to the composition of living organisms. Psychosomatic refers to the connection between the mind and bodily processes, emphasizing how psychological factors can affect physical health, but it does not pertain to physical substances like protoplasm. Thus, protoplasm is the correct term for the material that constitutes the essential substance of life in cells.