In his Disputed Questions on Truth q. 1 a. 1 arg. 3, St. Thomas Aquinas presents an argument against "that the true (verum) is exactly the same as being (ens)":

3. Things which differ conceptually [ratione or "in reason"] are so related to each other that one of them can be understood without the other. For this reason, Boethius says that the existence of God can be understood if for a moment we mentally separate His goodness from His existence. Being, however can in no way be understood apart from the true, for being is known only in so far as it is true. Therefore, the true and being do not differ conceptually.

Praeterea, quaecumque differunt ratione, ita se habent quod unum illorum potest intelligi sine altero: unde Boetius in libro de hebdomadibus dicit, quod potest intelligi Deus esse, si separetur per intellectum paulisper bonitas eius. Ens autem nullo modo potest intelligi si separetur verum: quia per hoc intelligitur quod verum est. Ergo verum et ens non differunt ratione.

I object to the major premise of this argument. For instance, in mathematics a twin prime and a prime are different; however, one cannot grasp what a twin prime is without understanding first what a prime number is. Thus, St. Thomas's argument does not seem to show that truth and being are not the same.

  • @EnlightenedFunky Not necessarily their existence, but at least it should be possible to understand the definition of twin prime without the definition of prime Oct 28, 2017 at 16:27
  • I just thought of something you said truth, and being so which one is truth, and being in your instance? So the truth, could be the prime, and the being the twin the prime. Oct 28, 2017 at 16:28
  • @EnlightenedFunky Truth and being are instances of the argument that two things that differ in reason ought to be understood without the other. My problem is with the general argument, where I use primes and twin primes as instances, and I don't know why it isn't a contradiction Oct 28, 2017 at 16:32
  • But aren't they also the same in some manner? I am just thinking about how your example has roots in each other, because a prime has root in twin primes, but does twin primes have roots in primes? Like can you define a prime using twin primes? Oct 28, 2017 at 16:40
  • @EnlightenedFunky That's a very interesting question! My intuition says you should not be able to, but I cannot prove it's not the case Oct 28, 2017 at 17:04

2 Answers 2


It doesn't seem your counterexample works. Do twin primes and prime numbers differ conceptually?

St. Thomas answers his objection in Disputed Questions on Truth q. 1 a. 1 ad 3:

3. “Something can be understood without another” can be taken in two ways. It can mean that something can be known while another remains unknown. Taken in this way, it is true that things which differ conceptually are such that one can be understood without the other. But there is another way that a thing can be understood without another: when it is known even though the other does not exist. Taken in this sense, being cannot be known without the true, for it cannot be known unless it agrees with or conforms to intellect. It is not necessary, however, that everyone who understands the formal notion of being should also understand the formal notion of the true—just as not everyone who understands being understands the agent intellect, even though nothing can be known without the agent intellect.

Ad tertium dicendum, quod aliquid intelligi sine altero, potest accipi dupliciter. Uno modo quod intelligatur aliquid, altero non intellecto: et sic, ea quae ratione differunt, ita se habent, quod unum sine altero intelligi potest. Alio modo potest accipi aliquid intelligi sine altero, quod intelligitur eo non existente: et sic ens non potest intelligi sine vero, quia ens non potest intelligi sine hoc quod concordet vel adaequetur intellectui. Sed non tamen oportet ut quicumque intelligit rationem entis intelligat veri rationem, sicut nec quicumque intelligit ens, intelligit intellectum agentem; et tamen sine intellectu agente nihil intelligi potest.

  • I read this but it only says that the Difficulty 3 is equivocative and reaffirms that "it is true that things which differ conceptually are such that one can be understood without the other". As written, this fails every time for genus vs species, as in primes and twin primes, on Aristotelian definitions one has to understand genus before understanding species, but they are conceptually different. It seems that he was sloppy and what he meant was not "differ conceptually" but rather something like "are conceptually disjoint". His example of goodness vs existence in Difficulty 3 is like that.
    – Conifold
    Oct 31, 2017 at 20:31

What you have quoted isn't actually Thomas's view. It is an objection against Thomas's view. Read the entire thing.

  • I think you are right. There is a section containing "Answers to Difficulties". Welcome to this SE! Dec 6, 2018 at 22:21

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