[circlecity]

"How can anyone really be strong on metaphysics? Isn’t it really just someone’s theory?"

No, it's primarily just applied logic. Is the logical law of non-contradiction (a metaphysical doctrine) "just someone's theory"?

[stuartcr]

Does it really apply to metaphysics?

[ConservativeDude]

s the logical law of non-contradiction (a metaphysical doctrine)”

I believe according to Aquinas, this is the first principle of metaphysical reasoning.....you are quite correct.