The notion of grades of perfection in things is one with which everyone is familiar. We naturally conceive some beings as higher upon the scale of reality than others; as having “more” reality, so to speak—not necessarily, of course, in the literal sense of size or quantity—than others; as being more perfect, nobler, of greater worth, value, dignity, excellence, than others. Thus we regard the infinite as more perfect than the finite, spiritual beings as nobler than material beings, man as a higher order of being than the brute beast, this again as surpassing the whole vegetable kingdom, the lowest form of life as higher on the scale of being than inorganic matter, the substance-mode of being as superior to all accident-modes, the actualized state of a being as more perfect than its potential state, _i.e._ as existing in its material, efficient and ideal or exemplar causes. The grounds and significance of this mental appreciation of relative values in things must be discussed elsewhere. We refer to it here in order to point out another scholastic aphorism, according to which the higher a thing is in the scale of actual being, and the more perfect it is accordingly, the more efficient it will also be as a principle of action, the more powerful as a cause in the production of changes in other things, the more operative in actualizing their passive potentialities; and conversely, the less actual a thing is, and therefore the more imperfect, the greater its passive capacity will be to undergo the influence of agencies that are actual and operative around it. “As passive potentiality,” says St. Thomas,(74) “is the mark of potential being, so active power is the mark of actual being. For a thing acts, in so far as it is actual; but is acted on, so far as it is potential.” Our knowledge of the nature of things is in fact exclusively based on our knowledge of their activities: we have no other key to the knowledge of what a thing is than our knowledge of what it does: “_Operari sequitur esse_”: “_Qualis est operatio talis est natura_”—“Acting follows being”: “Conduct is the key to nature”.