Beware the man of one book
The Feast of Thomas Aquinas

Today is the feast of Thomas Aquinas!  Do something reasonable in his honor!>

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)

Moderns have a tough time with this.  My granddaughter took A History of Philosophy out of the library and from curiosity I checked it.  It described the Cosmological Argument thusly: 

1. Everything has a cause
2. The universe exists
3. Therefore the universe has a cause, viz., God

and declared that this was demolished by the infinite regress: "What caused God?" 
But neither Aristotle, Aquinas, Scotus, nor any other realist(*) philosopher ever made such an argument.  Nor do their epigones.  This does not stop folks like, say, Richard Dawkins from a) misrepresenting the argument, b) "refuting" the misrepresentation, then c) objecting that those who patiently try to explain the actual argument to him are throwing up ad hoc defenses!  One thing you can say about the ol' Dumb Ox: On every question he considered, he laid out the strongest arguments against each one.  To rebut a staw man was very bad form in a medieval Question. 

But it is no surprise that the Cosmological Argument (as well as the others) have been misrepresented.  They are based on a metaphysic of nature that was rejected several centuries ago, and the very terms in which it is framed are no longer used in the same senses.  So this little summary needs a bit of an intro. 

I am only going to outline the argument.  The summary of the "Five Ways" that appears in Summa theologica, Pt. I, Q,2, art. 3: Or here: Respondeo dicendum quod Deum esse quinque viis potest... is itself an outlne of arguments already well-known and covered in more detail in other books.  (It may be that Dawkins and others have only skimmed this outline, said "Hunh?" and thought it was the sum and total.)  For a nicely readable modern discussion, in far more detail, see Ed Feser's Aquinas: A Beginner's Guide. 

Here goes.  Let's see if I can get it right. 
A. Preliminaries.
1. Motion is the reduction of potency to act; that is, X moves from being potentially something to being actually something.  For example, the motion of an apple in ripening from green to red; or the motion of a rose from bud to bloom.  A thing cannot potenially be anything at all.  The apple is not potentially an armadillo.  The potency/act business was a response to Parmenides; but since we have largely forgotten the questions Parmenides asked, Aristotle's answers seem a bit odd to us.  A very clear and direct example of the reduction of potency to act is what in modern lingo is called the collapse of the wave function, as when Schroedinger's cat is reduced from potentially alive or dead to actually alive or dead.  Local motion (a change from potentially over there to actually over there) is only one kind of motion.  A better translation of motion is change. 
2. A thing cannot change itself.  A newspaper is potentially hot, but it cannot make itself hot.  It wants a match, or something similar.  Change can be effected only by something actual.  The match is actually hot, and so can give hotness to the newspaper.  What is in potency to something cannot actualize that something, since in a real sense, it does not exist yet and therefore is powerless to change anything.  Caution: A whole thing may be moved by one of its parts, as a dog is moved by its legs, its legs by its muscles, etc..
3. A mover cannot impart to another what it does not already have.  This is often misunderstood.  The mover can contain the motion or change either a) formally -- that is, it directly possesses the form itself, as a burning match possesses the fire which it then imparts to the newspaper -- or b) eminently.    The unstruck match possesses fire eminently because the phosphorus is caused to ignite by friction.  In the same way your baby blue eyes [presuming you have such] are "in" your DNA even though not a single nucleotide on your double helix actually has baby blue eyes. 
4. Essentially ordered versus temporally ordered.  Aquinas did not believe that it could be proven scientifically that the world had had a beginning in time.  Therefore, he did not argue from time sequences of mover and moved.  A is essentially ordered to B if B not only receives its power to move C from A, but cannot act on C at all unless A is presently acting upon it.  That is, B is an instrument used by A to move C.  For example, the golfer's hands move the golf club.  Otherwise, the golf club would not be moving right now.  And if the hands were to stop moving the golf club, the club would stop moving.  It does not have the power to move the golf ball except as an instrument used by the hands.  This is independent of any details of "beginning to be" moved.  In a temporally ordered series, the prior movers are prior in time, not simply in logical priority.  If the match is ignited by a striker, the match would still burn even if the striker were to disappear from the universe after ignition.  The golf ball would continue to move even if the club disappeared after striking it. 

B. Argument.
1. Some things in the world are in motion [are changing].   This is evident to our senses.  (Aquinas always begins with empirical sense impressions.) 

2. Whatever is in motion (is changing) is put in motion (changed) by another,

2.1 You can't put something in motion toward X unless it is potentially X. 
2.2 But if it is potentially X, then it cannot be actually X.  (Something that is actually hot cannot be also potentially hot.)
2.3 Only something that is actually X [formally or eminently] can move something toward X.
2.4 Thus a thing cannot be at the same time both mover and moved. 

3. But if the mover is in motion, it must be itself put in motion by another, then this also must be put in motion, etc.
4. In an essentially ordered series of movers, this cannot proceed to infinity.  (If it is temporally ordered, it could do so, provided there were no Big Bang or other beginning to space-time.  In an essentially ordered series, "prior" means "logically prior" not "prior in time."  Essentially ordered movers are acting right now if the effect is right now.  They are not movers that acted before and are now gone.  The golf ball is moved by the club, which is moved by the hands, with are moved by the arms, which are moved (in golfing) by the shoulders) which are moved by the muscles, which are moved (contracted/relaxed) by the nerves, which are moved by the motor neurons in the brain, which are moved by something or other in brain chemistry, and so on back through the act of will, etc. etc. 

4.1 In such a series, later members have no independent causal power and are moved as an instrument by an earlier member.  
4.2 But the earlier member is itself an instrument of a still earlier member and has no independent causal power of its own. 
4.3 Therefore, there must be a first mover, because if there were not, none of the instrumental causes in the series would be capable of acting. If there were no first member, there would be no series at all.  

5. The first  mover must be entirely actual, with no potency ("Pure Act").  If it were potentially anything else, it could be moved and would not be the first mover.

5.1 Hence, the first mover is itself unmoved, and does not require, indeed cannot have, a prior mover. It just IS.  If it could talk it would call itself I AM. 

5.2 Therefore, the first mover is perfect, since to be perfected with respect to X is to be moved from potentially X to actually X and the first mover is Pure Act. 

 

6. There can be only one first mover.  If there were two, one would possess X which the other lacks; but then the other would be in potency to X, and would not be a first mover. 

6.1 Therefore, First Mover is the first mover of all essentially ordered causal chains.
6.2 Therefore First Mover is outside all causal chains, and thus outside the material universe. 

7. First Mover is immaterial.  Anything material is subject to change in various ways, a being of Pure Act cannot be.  [Prime Matter is Pure Potency, just as First Mover is Pure Act.] 

8. First Mover is eternal.  To come into being or pass out of being is to move from potency to act, and First Mover is Pure Act.  It just IS.
9. First Mover is all-powerful; that is, full of all powers, either formally or eminently.  First Mover is the common source of all attributes, including all powers, and must therefore contain those powers formally or eminently. 
9.1 Consequently, there is something in First Mover that is analogous to intellect and will in humans. 
9.2 Therefore, First Mover possesses "intellect and will" and is therefore a person.  From now on, we can say "He" (or "She" if you prefer: First mover is the source of male and female powers, and so must contain those powers in some sense.)
9.3 To possess intellect is to know.  To be perfect is to lack any limitations on an attribute.  Therefore, He is all-knowing. 
10. To be the source of everything is to be the source of all goods, and therefore He is all-good.  (Note the meaning of all-good here.)
10. An evil is a defectus boni, a "lack or deficit of a good."  It is not a thing in itself.  It is the absence of a feature rather than a feature itself.  A being of Pure Act cannot lack for anything, since He would then be in potency regarding that privation.