Aquinas is a very tight system, with careful logic. It is interesting that the possible objection to Aquinas are contained in his own careful analysis, in that he explains very carefully that belief & 'proof' requires certain assumptions in order to proceed. The only way to seriously dispute Aquinas is just shrug and declare that you don't agree to accept the same premises. He, of course, thought that his premises were 'obvious' and inescapable....but that is itself a different debate.