But we must also consider whether it is compatible with perfect knowledge:  for there is nothing to prevent some kind of imperfect knowledge from being sometimes with perfect knowledge.  Accordingly we must observe that knowledge can be imperfect in three ways:  first, on the part of the knowable object; secondly, on the part of the medium; thirdly, on the part of the subject.  The difference of perfect and imperfect knowledge on the part of the knowable object is seen in the “morning” and “evening” knowledge of the angels:  for the “morning” knowledge is about things according to the being which they have in the Word, while the “evening” knowledge is about things according as they have being in their own natures, which being is imperfect in comparison with the First Being.  On the part of the medium, perfect and imperfect knowledge are exemplified in the knowledge of a conclusion through a demonstrative medium, and through a probable medium.  On the part of the subject the difference of perfect and imperfect knowledge applies to opinion, faith, and science.  For it is essential to opinion that we assent to one of two opposite assertions with fear of the other, so that our adhesion is not firm:  to science it is essential to have firm adhesion with intellectual vision, for science possesses certitude which results from the understanding of principles:  while faith holds a middle place, for it surpasses opinion in so far as its adhesion is firm, but falls short of science in so far as it lacks vision.

Now it is evident that a thing cannot be perfect and imperfect in the same respect; yet the things which differ as perfect and imperfect can be together in the same respect in one and the same other thing.  Accordingly, knowledge which is perfect on the part of the object is quite incompatible with imperfect knowledge about the same object; but they are compatible with one another in respect of the same medium or the same subject:  for nothing hinders a man from having at one and the same time, through one and the same medium, perfect and imperfect knowledge about two things, one perfect, the other imperfect, e.g. about health and sickness, good and evil.  In like manner knowledge that is perfect on the part of the medium is incompatible with imperfect knowledge through one and the same medium:  but nothing hinders them being about the same subject or in the same subject:  for one man can know the same conclusions through a probable and through a demonstrative medium.  Again, knowledge that is perfect on the part of the subject is incompatible with imperfect knowledge in the same subject.  Now faith, of its very nature, contains an imperfection on the part of the subject, viz. that the believer sees not what he believes:  whereas bliss, of its very nature, implies perfection on the part of the subject, viz. that the Blessed see that which makes them happy, as stated above (Q. 3, A. 8).  Hence it is manifest that faith and bliss are incompatible in one and the same subject.