Is that found in St. Thomas' Summa anywhere, or is it from another source?
The topic touches upon Vatican II particularly since traditionalists have had disagreements about whether the documents of Vatican II as a whole should be rejected or if only parts of the body of documents can be accepted while other parts are condemned.
This comes to mind for me because of Fr. Ripperger's mention of it in his talk about the influence of music. He talks about movies for a time and comments on how sins violating the second commandment taint a film because they cannot be simulated.
Also, I do think of even persons themselves because of the struggle of the God-given goodness in our nature that is yet at the same time fallen due to original sin. How can we consider a person still good if they lack sanctifying grace too?
This all goes for anything else, but from I remember learning from Bp. Sanborn in his talk in commenting on Bp. Williamson's "mentevacantism," he says that the grayness, or what he perhaps meant is confusion, is only in our minds, but in the reality outside there are only black and white.
See Summa Theologica
I q. 48 a. 4 (https://isidore.co/aquinas/summa/FP/FP048.html#FPQ48A4THEP1) ("Whether evil corrupts the whole good?").
Bp. Sanborn and most sedevacantists subscribe to what A. V. Xavier da Silveira terms "Monolithic Infallibility" (cf. Salza & Siscoe (https://isidore.co/calibre/browse/book/5904) pp. 416-421), e..g., from his (https://isidore.co/calibre/browse/book/5906)The Theological Hypothesis of a Heretic Pope (https://isidore.co/calibre/browse/book/5906)
Quotethe same thing must be said in relation to the conciliar documents which do not fulfill the same conditions. ... when a Council does not intend to define dogmas, strictly speaking it can fall into errors. Such a conclusion follows from the symmetry existing between the pontifical infallibility and that of the Church, stressed by the First Vatican Council itself.
and "Monolithic Infallibility & Differences among Anti-progressivists (http://www.traditioninaction.org/HotTopics/f068_Silveira_2.htm):"
Quotethe notion of a monolithic infallibility inspires most of the sede-vacantists as well as the neo-conciliar supporters who attach dogmatic authority to Vatican II. This notion is also at the root of the doubts, perplexities and troubles that torment many Catholics.