Different philosophers took this proposal to imply different things.
Some advocated a wholesale scrapping of our ordinary language descriptions of mental states, such that, down the road, people might develop a whole new (and vastly more accurate) vocabulary to describe their own and others' states of mind.
Perhaps the weakest were those of the epistemological variety.
It has been claimed, for example, that because people have had (and still have) knowledge of specific mental states while remaining ignorant as to the physical states with which they are correlated, the former could not possibly be identical with the latter.
According to Smart (1959), "there is no conceivable experiment which could decide between materialism and epiphenomenalism" (where the latter is understood as a species of dualism); the statement "sensations are brain processes," therefore, is not a straight-out scientific hypothesis, but should be adopted on other grounds.
Occam's razor is cited in support of the claim that, even if the brain-process theory and dualism are equally consistent with the (empirical) facts, the former has an edge in virtue of its simplicity and explanatory utility.This still left the problem of explaining introspective reports in terms of brain processes, since these reports (for example, of a green after-image) typically make reference to entities which do not fit with the physicalist picture (there is nothing green in the brain, for example). "I see a green after-image") into what he described, following Gilbert Ryle, as "topic-neutral" language (roughly, "There is something going on which is like what is going on when I have my eyes open, am awake, and there is something green illuminated in front of me").To solve this problem, Place called attention to the "phenomenological fallacy"—the mistaken assumption that one's introspective observations report "the actual state of affairs in some mysterious internal environment." All that the Mind-Brain Identity theorist need do to adequately explain a subject's introspective observation, according to Place, is show that the brain process causing the subject to describe his experience in this particular way is the kind of process which occurs when there is actually something in the environment corresponding to his description. Where Smart diverged from Place was in the explanation he gave for adopting the thesis that sensations are processes in the brain. Over the years, numerous objections have been levied against Type Identity, ranging from epistemological complaints to charges of Leibniz's Law violations to Hilary Putnam's famous pronouncement that mental states are in fact capable of being "multiply realized." Defenders of Type Identity have come up with two basic strategies in response to Putnam's claim: they restrict type identity claims to Place accepted the Logical Behaviorists' dispositional analysis of cognitive and volitional concepts. But it was not until David Armstrong made the radical claim that mental states (including intentional ones) are identical with physical states, that philosophers of mind divided themselves into camps over the issue.Occam's razor also plays a role in the version of Mind-Brain Type Identity developed by Feigl (in fact, Smart claimed to have been influenced by Feigl as well as by Place).On the epiphenomenalist picture, in addition to the normal physical laws of cause and effect there are psychophysical laws positing mental effects which do not by themselves function as causes for any observable behavior.The fact that Smart himself now holds that all mental states are brain states (of course, the reverse need not be true), testifies to the influence of Armstrong's theory.Besides the so-called "translation" versions of Mind-Brain Type Identity advanced by Place, Smart, and Armstrong, according to which our mental concepts are first supposed to be translated into topic-neutral language, and the related version put forward by Feigl, there are also "disappearance" (or "replacement") versions.As initially outlined by Paul Feyerabend (1963), this kind of Identity Theory actually favors doing away with our present mental concepts.The primary motivation for such a radical proposal is as follows: logically representing the identity relation between mental states and physical states by means of biconditional "bridge laws" (e.g., something is a pain if and only if it's a c-fiber excitation) not only implies that mental states have physical features; "it also seems to imply (if read from the right to the left) that some physical events..non-physical features." In order to avoid this apparent dualism of properties, Feyerabend stressed the incompatibility of our mental concepts with empirical discoveries (including projected ones), and proposed a of our existent mental terms.