In about 485 B.C., the Greek philosopher, Parmenides, presented an argument that he purported was proof that everything is the same. "All is one," was his conclusion. I must admit that no matter how counter-intuitive such claims may seem, I often get tripped up by the logic of these kinds of arguments, until I really think through them. Parmenides basically argues as follows:
1. If there were two or more beings, they would have to differ.
2. A being differs either by being or non-being.
3. Being is that which makes them identical.
4. Hence, differences cannot be by being.
5. Non-being is nothing.
6. Hence, to differ by non-being is to not differ at all.
7. Therefore, there is only one being.
Upon examination, it is often difficult for one to tell which premise is incorrect. And, if the premises are true, so is the conclusion. However, if one is careful to read (3) and trace its implications, then it becomes apparent that Parmenides is begging the question. For, he assumes that all being is identical in order to conclude that nothing can differ by being.
The solution to the problem is likely what Thomas Aquinas suggested - namely, that there are different types of being. This "complexity of being" allows the objector to rightfully reject Parmenides' argument. This makes all the more sense once we consider the previous arguments for God's existence that have been considered. God is Pure Act (or, Pure Being), whereas finite, changing beings are composed of both actuality and potentiality. The varying degrees of potentiality per attribute are what constitute complex being.
For more on this, see: Norman Geisler, Thomas Aquinas: An Evangelical Appraisal, Wipf and Stock Publishers, 2003, pp. 91-101.