 Suppose some things, the \(X\)s, are alike in a certain way: they share some feature, \(F\). The theory of Forms says that if this is so then there is some form, \(F\)-ness, in which the \(X\)s each participate and in virtue of which they have that shared feature. The theory of Forms also says that Forms are self-predicated: the Form of the good is good, the Form of largeness is large, etc. So the Form of \(F\)-ness is \(F\). In that respect, then, it is like each of the \(X\)s. So not only are the \(X\)s all alike in a certain way, the \(X\)s and \(F\)-ness are all alike in a certain way. And so there must be some Form in which each of the \(X\)s and \(F\)-ness participate, in virtue of which they have this shared feature. However, this Form cannot be \(F\)-ness, because the Form must be distinct from the things that have that character that participate in it, so the \(X\)s and \(F\)-ness must participate in a new form \(F_1\)-ness. But \(F_1\)-ness will, just like the \(X\)s and \(F\)-ness, be \(F_1\), given that Forms self-predicate, and so the \(X\)s, \(F\)-ness and \(F_1\)-ness are all alike in a certain way … and so on. We are off on an infinite regress.