This same principle of religion and theology had been already invoked in a philosophical context by certain sceptics, as a weapon against the presumptuous claims of human knowledge. Francisco Sanchez, for example, in his _Quod nihil scitur_ (1581), in discussing the difficulty of knowing the nature and powers of the soul, had observed that if man could have this knowledge in a perfect degree he would be like God, or rather he would be God himself: since it is impossible "that one should know perfectly things which he has not created, nor could God have created things of which he had not perfect foreknowledge, nor ruled them when created: he himself therefore, being alone the perfect wisdom, knowledge, and intellect, penetrates all things, is wise concerning all things, knows all things, and understands all things, because he is all things and in all things, and all things are he and in him" (_perfecte cognoscere quis quae non creavit, nec Deus creare potuisset nec creata regere quae non perfecte praecognovisset: ipse ergo solus sapientia cognitio intellectus perfectus omnia penetrat omnia sapit omnia cognoscit omnia intelligit, quia ipse omnia est et in omnibus, omniaque ipse sunt et in ipso_).[1] But Sanchez appeals to this thought only in passing, and without grasping its philosophical import or realising that his hand was resting upon a treasure; while Vico for the first time drew from the praise of the infinite power and wisdom of God, and from their contrast with the limited faculties of man, the universal principle of his theory of knowledge, that the condition under which a thing can be known is that the knower should have made it, that the true is identical with the created: _verum ipsum factum._ This, he explained, is precisely what is meant by saying that science is to know by causes, _per causas scire._ Since a cause is that which has no need of anything external in order to produce its effect, it is the genus or mode of a thing: to know the cause is to be able to realise the thing, to deduce it from its cause and create it. In other words, it is an ideal repetition of a process which has been or is being practically performed. Cognition and action must be convertible and identical, just as with God intellect and will are convertible and form one single unity. Now once this connexion of the true with the created is recognised as the ideal, and indeed, since the ideal is the truly real, as the true nature of science, the first consequence of such a recognition must be that science is unattainable to man. If God created the world, he alone knows it _per causas,_ he alone knows its genera or modes, he alone possesses scientific knowledge of it. Did man make the world? Did he make his own soul?