The identity theory is the claim that the mind is identical with the brain, and that every mental state has an equal physical state. One criticism of the identity theory is the fact that it focuses only on the ability of the human mind to produce thought and reason, and doesn't focus on only the function of producing thought. For this reason, functionalists argue that the identity theory is inferior because it fails to allow other minds, like alien minds, to technically produce thoughts under their definition. Secondly, the spacial position argument is often used against the identity theory. The identity theory argues that every mental state has a physical equivalence, so it would argue that every mental state may be located. However, it is impossible to locate a thought. For example, if you were to say, "Mary is looking for her mother", that thought could not be located by answering "Mary's whole body", because that is not where the thought exists. In the same way, depression is a pain that cannot be located in the brain and isn't necessarily susceptible to c-fiber stimulation.