Which philosophy aligns most closely with the notion that only physical matter exists?

Materialism is a philosophy that asserts that only physical matter is real and that everything that exists, including thoughts, feelings, and consciousness, can be explained in terms of physical processes and properties. This viewpoint posits that the material world is the fundamental basis of reality, emphasizing that non-material concepts such as ideas or emotions ultimately derive from physical states or interactions.

In contrast, idealism focuses on the mental or spiritual aspects of existence, emphasizing that reality is shaped by ideas and consciousness. Naturalism, while closely related to materialism, extends to include natural laws and the processes governing nature, asserting that everything can be explained through natural phenomena without invoking supernatural elements. Empiricism, on the other hand, stresses the importance of sensory experience as the foundation of knowledge but does not exclusively assert that only physical matter exists; it is concerned with how we acquire knowledge rather than the nature of reality itself.

Therefore, materialism most directly encompasses the idea that physical matter constitutes the entirety of existence, making it the correct answer.