Beware the man of one book
The Feast of Thomas Aquinas
Today is the feast of Thomas Aquinas! Do something reasonable in his honor!

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Some Thomistic Quotes:
(I've added sources where I know them, but did not chase them down. If anyone knows the source text, let me know.)
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)
A miscellany of quotes:
Some of the following seem a bit too colloquial, but the Internet cannot be wrong, can it?
Good can exist without evil, whereas evil cannot exist without good.
A man has free choice to the extent that he is rational.
All the efforts of the human mind cannot exhaust the essence of a single fly.
If the highest aim of a captain were to preserve his ship, he would keep it in port forever.
Because of the diverse conditions of humans, it happens that some acts are virtuous to some people, as appropriate and suitable to them, while the same acts are immoral for others, as inappropriate to them.
Friendship is the source of the greatest pleasures, and without friends even the most agreeable pursuits become tedious.
How is it they live in such harmony the billions of stars - when most men can barely go a minute without declaring war in their minds about someone they know.
It is requisite for the relaxation of the mind that we make use, from time to time, of playful deeds and jokes.
Sorrow can be alleviated by good sleep, a bath and a glass of wine.
To bear with patience wrongs done to oneself is a mark of perfection, but to bear with patience wrongs done to someone else is a mark of imperfection and even of actual sin.
It is possible to demonstrate God's existence, although not a priori, yet a posteriori from some work of His more surely known to us.
Which Brings Us to Today's Bonus Feature
The First Proof of the Existence of God
(The Cosmological Argument)