Mereological nihilism is the view that composite objects don't really exist. All that truly does exist (besides God and other immaterial things, granting the arguments from natural theology are consistent with mereological nihilism, and they are, and are also correct) are the most fundamental particles, whether they be quarks or strings, or whatever.
The first problem with mereological nihilism is that it is highly counter-intuitive. Of course, this point isn't enough the refute it, but it's a start. Hylomorphism is the Aristotelian view that composite things do exist, and they exist as a combination of matter and form. Forms are universals that are instantiated within the thing itself. This is why we are able to compare stars with one another. Our sun is one of the smaller stars, relatively-speaking. Without form, or at least the concept of form, all that we would perceive are clumps of fundamental particles, and nothing would appear distinct from anything else.
The mereological nihilist could, then, adopt conceptualism. Forms exist on this view, but they only exist as mental concepts. The difficulty with this is that what is the mind (on a strong version of mereological nihilism) other than an arrangement of fundamental particles? How can an arrangement of fundamental particles think? This, of course, brings us to the mind-body problem, which is beyond the purview of this post.