From “Demonstrating in Natural Philosophy” by Melvin Glutz:

A man may have only opinion of a conclusion that is objectively a scientific and necessary proposition. One may see the proposition as impossible to be otherwise, and thus have science. Another may think it possible to be otherwise. The root of his difficulty is in the understanding of the immediate propositions. The first man sees them as immediate; the other cannot see them as such, so he is barred from scientific knowledge of the conclusion. A corollary to this possibility of a scientific demonstration being known only opinionatively is the need to understand demonstrative theory, and this especially for beginners in philosophy. If one does not understand the process of demonstration, he will indeed learn philosophy: he will know the doctrines of Thomism and perhaps even teach them. But if he does not recognize and understand the demonstrations, his knowledge will not be scientific, but opinionative and always vacillating and unstable. He will therefore be tempted to look around for novel opinions, or to confine himself to the historical approach to philosophy, or to become a mere cataloguer of opinions.