Continuing from where we left off in Part One, Etienne Gilson describes the bedrock of Thomist realism in his Le Réalisme Methodique, as reproduced by E.L. Mascall in his The Openness of Being. Included are comments by Mascall in italics.
Knowledge presupposes the presence of the thing itself to the intellect, and we do not have to postulate, behind the thing that is in the thought, a mysterious and unknowable duplicate, which would be the thing of the thing as it is in thought but, in thought, apprehending the thing as it is.
Taine performed a great service for good sense when he defined a sensation as a true hallucination, for he showed where logic necessarily lands idealism. Sensation is what a hallucination becomes when this hallucination isn't one. We must not let ourselves be impressed by the famous 'errors of the senses' or be surprised by the enormous hoo-ha that the idealists make of them; idealists are people for whom the normal can only be a particular case of the pathological.
E.M: Gilson refuses to admit the accusation that realists are committed by their doctrine to posing as infallible; quite to the contrary:
We are simply philosophers for whom truth is normal and error is abnormal; this does not mean that truth is any earier for us to achieve that is, for example, perfect health. The realist does not differ from the idealist in being unable to make mistakes, but primarily in the fact that, when he does make mistakes, it is not because thought has erred through being unfaithful to itself but because knowledge has erred through being unfaithful to its object. But, above all, the realist makes mistakes only when he is unfaithful to his principles, while the idealist avoids them only in the degree in which he is unfaithful to his.
E.M: And finally, it is the idealist, not the realist, who takes the mystery out of existence and claims to know everything that there is to know:
To say that all knowledge consists in grasping the thing as it is does not in any way mean that knowledge exhausts the content of its object in one single act. What knowledge grasps of an object is real, but the real is inexhaustible, and even if the intellect had discerned all its details it would still be up against the mystery of its very existence. It was the idealist Descartes who believed that he could grasp the reality infallibly and at one fell swoop; Pascal, the realist, knew how naive this pretence of the philosopher was... The virtue proper of the realist is modesty concerning his knowledge, and even if he does not always practise it, he is committed to it by his profession.
[E.Gilson, Le Réalisme Methodique, pp. 87ff cited in E.L. Mascall, The Openness of Being, pp. 94-95]