First principles of practical reason?

What does it mean to say that these first principles of natural law are known to all -are self-evident truths of the practical reason?

One such law is, “do good and avoid evil”. But this proposition seems far from self-evident to me -I can easily say that one should do evil and avoid good w/o noticing a contradiction and what can be admitted w/o contradiction is not self-evident in the strict sense.

Any help here?

It means they are evident on a purely natural level, i.e., without the supernatural helps of grace, faith, revelation, etc. St. Thomas Aquinas says natural law is something pertaining to reason: “the natural law is something appointed by reason” (What is the natural law?).

Or the principle of non-contradiction, which St. Thomas Aquinas formulates as (, I-II q. 94 a. 2Summa Theol. c.):

Wherefore the first indemonstrable [For why, see the quote below from St. Thomas’s commentary on Aristotle’s *Metaphysics.]

principle is that “the same thing cannot be affirmed and denied at the same time,” ** which is based on the notion of “being” and “not-being”: and on this principle all others are based, as is stated in Metaph. iv, text. 9. Now as “being” is the first thing that falls under the apprehension ** simply, so “good” is the first thing that falls under the apprehension of the practical reason, which is directed to action: since every agent acts for an end under the aspect of good. Consequently the first principle of practical reason is one founded on the notion of good, viz. that “good is that which all things seek after.” Hence this is the first precept of law, that “good is to be done and pursued, and evil is to be avoided.” All other precepts of the natural law are based upon this: so that whatever the practical reason naturally apprehends as man’s good (or evil) belongs to the precepts of the natural law as something to be done or avoided.

Are you asking why “good is that which all things seek after” rather than evil, which things can indeed seek after to a certain degree? The reason is simple: things seeking evil tend to lose being, and in so doing they lessen their being a thing. If a being seeks evil completely, it ceases to exist. So all things cannot seek evil because then they would be non-things, and this would violate the principle of non-contradiction.

So, why is the principle of non-contradiction indemonstrable? Commenting on Aristotle’s Metaphysics, St. Thomas writes (Commentary on Aristotle’s Metaphysics, book 4,lecture 6 [607.]):

[Aristotle] says, first, that certain men deem it fitting, i.e., they wish, to demonstrate this principle; and they do this “through want of education,” i.e., through lack of learning or instruction. For there is want of education when a man does not know what to seek demonstration for and what not to; for not all things can be demonstrated. For if all things were demonstrable, then, since a thing is not demonstrated through itself but through something else, demonstrations would either be circular (although this cannot be true, because then the same thing would be both better known and less well known, as is clear in Book I of the Posterior Analytics

[cf. St. Thomas’s lecture: [/COLOR]Expositio Posteriorum, lib. 1 l. 7]), or they would have to proceed to infinity. But if there were an infinite regress in demonstrations, demonstration would be impossible, because the conclusion of any demonstration is made certain by reducing it to the first principle of demonstration. But this would not be the case if demonstration proceeded to infinity in an upward direction. It is clear, then, that not all things are demonstrable. And if some things are not demonstrable, these men cannot say that any principle is more indemonstrable than the above-mentioned one.

To choose evil is to reject reality because good is the positive - and evil the negative - aspect of reality.

But wait, if the natural law (it’s first principles at least) are not innate ideas, then it is possible not to know them -to be ignorant of the natural law. But this is impossible since the natural law can’t be taken away from humans. So either someone could not know the natural law or it is innate in man -either position seems untenable.


Someone could be ignorant as to the the conclusions drawn from such principles. Aquinas uses the example of the Germans who did not think theft was wrong (at least in the writings of Caesar).

I guess the question you’re asking is how one knows the moral principles. I don’t think Aquinas meant on the cognitive level, although one certainly can gain that knowledge ( I didn’t know them cognitively until I studied Aquinas).

I think we should first come to understand what we mean by nature, and the most given definition by Aquinas is “internal principle of motion.”

So we “know” the natural law simply by being determined in the natures we have. Someone never knowingly chooses evil, lest they see some good in it. The adulterer does not choose to cheat on his wife because it is evil, but because the pleasure with another woman is desired.

So if you walk through the precepts, they are basically how we behave simply based on the internal principle motion within us.

We choose good (evil is only chosen inasmuch as it is good, albeit a lesser one).

We preserve ourselves every day by eating and the like. Thus we “know” the natural law by abiding by it.

We seek to preserve the species by being attracted to the other sex and, having and rearing children.

We seek the rational goods by wanting to live in society (name one human being who exists without society), and by seeking to know the truth. Just listen to a 3 year old some time to see this part of our natures – they never stop asking “why?”

So, in short, we "know’ the natural law by being determined to certain behaviors based on our natures (internal principle of motion.)

Even pagans attested to the natural law.

The world is in chaos because of this dichotomy between the precepts of natural moral law and the flight from reason. The ancient Egyptians and the pagan Cicero, before Christ, acknowledged the natural moral law: Cicero (died 43 B.C.) wrote in De Republica, 3.22: “True law is right reason in agreement with nature. It is of universal application, unchanging, everlasting. We cannot be freed from it by Senate or people. This law is not one thing at Rome and another at Athens, but is eternal and immutable, valid for all nations and for all times. God is the Author of it, its promulgator, and its enforcing judge. Whoever is disobedient to it is abandoning his true self and denying his own nature.” The result of course is plain for all to see through contraception (and abortion) – the demographic winter over Europe and fast approaching in the rest of the world.

It might be helpful to think of the first principle as a procedural guideline. It’s a necessary proposition that, as was said above, is self-evident in that denying it involves a contradiction.

“Seek to do good and avoid evil” (or however you formulate this principle) is a principle of practical reason, NOT a principle of moral reason. The first principle of practical reason doesn’t tell you what is good/bad. Your answer to those evaluative terms must come from moral reasoning.

Think about the famous line of Milton’s anti-hero in Paradise Los: “Evil, be thou my good.” I think most of us would agree that we hold other things to be good than the speaker of the above line, and yet, we are all bound by the first principle of practical reason.

As only a formal principle, we must supply what is bad and good. The fact that the words good and bad already respectively are defined as being desirable and undesirable makes this principle necessary.

Sure; this is one of my favorite subjects; I hope I can help.

The first principle of practical reason, which can be summed up as you do (“do good, avoid evil”) is not a commandment that one can either follow or disobey. It is also not an opinion that one might or might not have. It is something that actually guides every single one of each person’s actions.

For us, for men, to consider something as good means to consider it worthy of pursuit. If you are acting, then you are after something, and therefore you consider that something as good. There is a necessary connection between considering something good and considering it worthy of pursuit; this is what the principle states.

So you see, the first practical principle is merely that which places us in the “realm” of finality, in the realm of action.

Compare it with the first speculative principle (the principle of non-contradiction). We always think according to the principle of non-contradiction. It is under the light of that principle that we think; otherwise we could not think. Likewise, it is under the light of the first practical principle that we act, otherwise we could not act.

Yes, there is a third option. The principles of natural law arise in the individual as soon as he begins to receive sensory data and act upon the world with the use of his reason. It doesn’t matter which bits of sensory experience you have, since the first principle deals with such basic stuff of reality that any experience will be enough for the human intellect to discover it.

The mere fact of being alive and perceiving the world leads one to knowing some basic things such as that something cannot be and not be in the same respect at the same time, and that good is to be done and evil avoided. St. Thomas refers to this as arising from the very light of the agent intellect.

What is the “principle of moral reason,” then? Thanks

The function of the first principle of practical reasoning is, as was mentioned above by several people, to direct human action toward the desirable. Again, what is desirable is not specified by the first principle of practical reasoning. Thomas talks about goods or ends of human actions, and points out that there are some human goods (God is the ultimate, the Summum Bonum) which are goods to which human action is directed. Thomas doesn’t give a full list of those goods but limits himself essentially to saying things like “life and the like” or “things of this kind.” Some Catholic theologians and philosophers have tried to offer a complete list of these integral humans goods.

Assume now that you have a list of these human goods. The first principle of morality is essentially making sure that we are able to distinguish between the different options of choices that are morally good/bad. John Finnis has said that the first principle of morality in Aquinas (“the neighbour-as-oneself principle”) acts for Aquinas like the categorical imperative does for Kant. Once you have the first principle down, there are other directives that further specify how you go about choosing in a morally responsible manner. Grisez calls these “modes of responsibility.”

Morality flows from the principles of practical reason.

So, the first is that “good is to be done, evil avoided”. And we know as self-evident truths that, for instance, knowledge is a good thing, and having friends with whom to talk is a good thing. From these follow moral precepts (which are not to be understood as commandments weighing down on someone’s conscience, but as things which we in fact want to do; otherwise they wouldn’t be precepts for us), such as that we should try to acquire knowledge about things that are important to us, that we shouldn’t lie to others (to conclude this we need an extra piece of knowledge: that other persons are no more and no less objectively important as ourselves), that we should seek to treat others well, etc.

Now we have morality. Notice that morality thought of in this way does not have “duties”, obligations that just are and ought to be obeyed contrary to our best knowledge. Obligations arise, in this conception, from our relation to the ultimate end, that is happiness. So, what morality says is basically “If you want to be happy, do such and such”. Well, everyone wants to be happy. Therefore, they must do such and such. And if they don’t, well, they’ll have to live with the consequence of their choices (unhappiness).

Really? I thought he did specify it by saying “good is to be done and pursued, and evil is to be avoided.” (, I-IIª q. 94 a. 2Summa Theologiæ co.). So good is desirable. “Good” is not an empty word; the “good is that which all things seek after.” Do we not know that to “which all things seek after?” Why does it seem you are setting up a false dichotomy between “practical reason” and “moral reason?”

Okay, here you go:[bibledrb]1 Timothy 4:4[/bibledrb]

I would think Aquinas would say the first principle of morality is what I quoted above. Creatures—including our neighbors—are good only because they have being from the Supreme Good. “Good” and “being” are the same.

Thanks for the help

Look again at the quote I put up from Milton: “Evil, be thou my good.” The devil has identified what will be good for him; Milton’s beautiful verse can sometimes distract from the nice irony of the line. The devil is at once subverting and upholding “good.” He is stating that he will pursue the good, but now, he identifies what we would consider evil as being his good. That is the first principle of practical reasoning. There is no false dichotomy to be established between the first principle of practical reasoning and that of morality. You must have both. The first principle of morality and the specified directives that follow from that first principle are essentially what Thomas described as prudence.

Try this: the first principle of practical reasoning is specified by identifying the human goods for which we ought to act. The first principle of morality is specified through the modes of responsibility. Practical reasoning can give you the targets, and the principle of morality and the specified modes of choosing among competing goods allows you to make a moral choice given that you have identified the goods.

The goods/ends for which we act must be identified. It is all good and well for a faith believer to say that “God is good” (indeed, He is) and to just follow the Commandments, but the natural law is supposed to allow people on this earth to determine right/moral action through the God-given power of human reason without explicit appeal to revelation or divine authority.

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