1. Beware the man of one book.
2. It is better to illuminate than merely to shine, to deliver to others contemplated truths than merely to contemplate.
3. By nature all men are equal in liberty, but not in other endowments.
(Not even Thomas Jefferson said it better....)
4. If to provide itself with a king belongs to the right of a given multitude, it is not unjust that the king be deposed or have his power reduced by that same multitude if, becoming a tyrant, he abuses his royal power.
(On Kingship, I:6) (Not even Thomas Jefferson said it better....)
5. We marvel at something when, seeing an effect, we do not know the cause. And since one and the same cause is at times known to certain people and not to others, it happens that some marvel and some do not.
(On the truth of the catholic faith against the gentiles)
6. Since Holy Scripture can be explained in a multiplicity of senses, one should adhere to a particular explanation only in such measure as to be ready to abandon it, if it be proved with certainty to be false; lest Holy Scripture be exposed to the ridicule of unbelievers, and obstacles be placed to their believing.
(Summa theologica, Part I, Q. 68, art. 1)
7. Nature is nothing but the plan of some art, namely a divine one, put into things themselves, by which those things move towards a concrete end: as if the man who builds up a ship could give to the pieces of wood that they could move by themselves to produce the form of the ship.
(Commentary on Physics II.8, lecture 14, no. 268)
8. Sciences are differentiated according to the various means through which knowledge is obtained. For the astronomer and the physicist both may prove the same conclusion—that the earth, for instance, is round: the astronomer by means of mathematics (i.e., abstracting from matter), but the physicist by means of matter itself.
(Summa Theologiae, Part I, Q. 1, art. 1)
9. The suppositions that these astronomers have invented need not necessarily be true; for perhaps the phenomena of the stars are explicable on some other plan not yet discovered by men
llorum tamen suppositiones quas adinvenerunt, non est necessarium esse veras: licet enim, talibus suppositionibus factis, apparentia salvarentur, non tamen oportet dicere has suppositiones esse veras; quia forte secundum aliquem alium modum, nondum ab hominibus comprehensum, apparentia circa stellas salvantur.
(De coelo [On the heavens], II, lect. 17)
10. The theory of eccentrics and epicycles is considered as established, because thereby the sensible appearances of the heavenly movements can be explained; not, however, as if this proof were sufficient, forasmuch as some other theory might explain them.
Sicut in astrologia ponitur ratio excentricorum et epicyclorum ex hoc quod, hac positione facta, possunt salvari apparentia sensibilia circa motus caelestes, non tamen ratio haec est sufficienter probans, quia etiam forte alia positione facta salvari possent.
(Summa theologica, I, Q.32, art.1)
11. Practical sciences proceed by building up; theoretical sciences by resolving into components.
Necessarium est enim in qualibet operativa scientia ut procedatur modo compositivo, e contrario autem in scientia speculativa necesse est ut procedatur modo resolutivo, resolvendo composita in principia simplicia.
(Sententia libri Ethicorum [Commentary on the Nicomachean Ethics], Bk. I, chap. 3, no. 4)
I like number 10 given what lay in the future for Bellarmine, Galileo and the Church.