Ed Feser says the following in his Aquinas:

The Greek philosopher Parmenides (c. 515–450 B.C.) notoriously held that change is impossible. For a being could change only if caused to do so by something other than it. But the only thing other than being is non-being, and non-being, since it is just nothing, cannot cause anything. Hence, though the senses and common sense tell us that change occurs all the time, the intellect, in Parmenides’ view, reveals to us that they are flatly mistaken.

I’ve italicised the premise that I’m having trouble with. ‘It’ seems to refer to something other than being. But why suppose that a thing can change only if caused to do so by something other than being? What instead seems true is that a thing can change only if caused to do so by something - perhaps something other than the thing that’s changing (though even this doesn’t seem right, since the change may be caused by a part of the thing that’s changing). There doesn’t seem to be any reason whatsoever for accepting this premise.

What’s the argument for it?