I have been at considerable pains to explain and vindicate this argument in my Ethics and Natural Law, pp. 13-21. The alternative to the acceptance of it is the view of Professor Stewart, -- and, no doubt, of Aristotle: -- "The theôrêtikos bios is an ideal: it cannot be realised by man, for he is concrete. But the effort to realise it, so far as possible, is all important in human life. The effort to realise it co-ordinates man's powers, it gives him élan, and carries him on to the attainment of many things within his reach, which he would not otherwise aspire to" (Stewart's Notes on Nicomachean Ethics, II, 448). Is man then a lusus naturae, who wins an insufficient pittance in repeated doles by ever asking for more? Is this what Ecclesiastes xii calls all man? We have then the fourth petition of the Lord's Prayer granted to the rejection of the second, which scarcely looks like the fulfilment of the third. We have daily bread, but no kingdom come. We have the race progressing indefinitely, but all individual progress ending at no long time in a plunge into nothingness. Is not the case the same with all other animal life and with the whole vegetable world? To be sure it is, but man alone knows it, and his knowledge is his misfortune.
Of God and His Creatures: 3.48