Functionalism is more or less a type of physicalism, yet is frequently classified as a distinct theory (Van Gulick 1). Functionalism holds that each possible mental state (emotion, etc.) has a direct causal relationship to another mental state (Marlow 1). This theory is very popular among computationalists, or those who believe that the brain is a biological implementation of a computer.

Functionalism distinguishes itself from physicalism because it is solely concerned with the effective functions of the brain and its organization.

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Critique of this theory

Again, it could be argued that functionalism is a very simplistic theory. While functionalism does support the idea that conscious artificial intelligence could very easily become a reality, it doesn't actually address the idea of consciousness itself as much as it should. In fact, it fails completely at addressing the act of experiencing, and instead focuses on what people do as a result of said experimenting.

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Citation

Marlow, Kristian. “What is Consciousness?” Psychology Today Sussex Publishers, 01 Mar. 2013. Web. 23 Nov. 2015.

Van Gulick, Robert. "Consciousness." The Stanford Encyclopedia of Philosophy. Ed. Edward N. Zalta. The Metaphysics Research Lab, 14 Jan. 2014. Web. 24 Nov. 2015.