St. Thomas explained the meaning of his formula quite clearly, especially in the Summa Theologica, Part I. question xvi. art. 3. Here he asks whether the truth and the reality are convertible, _utrum verum et ens convertantur;_ to which he replies as follows: "that as the good is of the nature of the desirable, so the truth has the nature of knowledge. But in so far as a thing has existence in itself, thus far it is knowable. And for this reason it is said in _De anima,_ Bk. III. text. 37 (431 b 21) that 'the soul is in a sense all things' according to sense and intellect. And hence as the good is convertible with the existent, so is the true. But yet as the good adds to existence the nature of the desirable, so also the truth adds a reference to the intellect." (_Quod sicut bonum habet rationem appetibilis, ita verum habet ordinem cognitionis. Unumquodque autem in quantum habet de esse, in tantum est cognoscibile. Et propter hoc dicitur in_ 3 _de Anima, text._ 37, _quod 'anima est quodammodo omnia' secundum sensum et intellectum. Et ideo sicut bonum convertitur cum ente, ita et verum. Sed tamen sicut bonum addit rationem appetibilis supra ens, ita et verum comparationem ad intellectum._) Nothing then can be known except what exists, and nothing can exist but what is good: existence, truth and goodness are all convertible. Thus, too, things are called good in so far as they correspond to the idea in their Creator's mind. "Each single thing partakes of the truth of its own nature in so far as it imitates the knowledge of God, like an artefact in so far as it agrees with the art": "the knowledge of God is the cause of things": "the knowledge of God is the measure of things." (_Unumquodque in tantum habet de veritate suae natura, in quantum imitatur Dei scientiam sicut artificiatum in quantum concordat arti_ I. xiv. 12. _Scientia Dei est causa rerum_ I. xiv. 12. _Scientia Dei est mensura rerum_ I. xiv. 12.) But truth and goodness, the objects of intellect and will respectively, if on the one hand they are "convertible in reality," _convertentur secundum rem,_ on the other they are "distinguishable in thought," _diversificantur secundum rationem_ (I. lix. 2). What have these thoughts in common with Vico's idea that the condition of knowing a truth is to create it? In fact, what is here stated is that the condition of making a thing is to know it, or as St. Thomas says in the same place (I. xiv. 8) in St. Augustine's words (_De Trinitate_ xv. 13) "_Universas creaturas et spirituelles et corporales non quia sunt ideo novit Deus, sed ideo sunt quia novit._" (God does not know all His creatures corporeal and spiritual because they exist: but they exist because He knows them.)