I sometimes hear people describe humans as a “bunch of molecules,” or something to that effect. For example, “Humans are ultimately just a bunch of molecules.” This is false, unless a strong emphasis is on ‘bunch’.
The reason is that the most obvious aspect of humans is our subjective experience. (‘Experience’ here does not mean ‘experience’ as in memory and practical skills developed, it means experience as in “I am having an experience right now.” ‘Subjective’ here is not used in the sense of something distorted due to personal bias, but rather as in a mode of being.)
This subjective experience is complex, unified, and real. For example, I might have a visual experience of a meadow. There are many entities in the visual experience (grass here, sky there, and so on), so it is complex in a certain sort of way. These things are in a visual experience, so they are unified. The experience itself is real (i.e., it exists), as opposed to it being an arbitrary grouping of external things (such as may be the case when we talk of a ‘bunch of molecules’).
There is something which must account for the unity of an experience. To one who believes that humans are nothing but ‘molecules’, there does not appear to be anything at hand that is a good explanatory fit.
This problem interconnects with several other problems, including: the unity of experience through a series, subjective being, and the qualitative nature of experience. The above problem of the unity of an experience is symptomatic – someone who describes humans as a ‘bunch of molecules’ will probably be unable to adequately answer any of these problems. This is why someone like Colin McGinn says that the qualitative nature of experience is inexplicable. McGinn is better than many, in that he acknowledges that there is a difficult problem(s) at hand, where the conceptual resources of physicalism do not seem able to afford an answer.
It might be important to note that the concept ‘physical’ has changed dramatically over the past, say, 400 years. With our current conception of ‘physical’, my working guess is that McGinn is right that the problem of the subjective self is an insoluble mystery for physicalists. That is to say, if one wishes to remain a ‘physicalist’ and solve these problems, something must change within one’s conceptualization of what ‘physical’ things can include.