With these ultimate principles Cicero, in his _De Natura Deorum_, approaches the questions of the existence of God and of the human soul. The bias of his own nobler nature led him to hold fast these two vital truths, but he is fully aware that in attempting to prove them the Stoics have used arguments which are not convincing. In the Tusculan disputations [73] he acknowledges the necessity of assuming one supreme Creator or Ruler of all things, endued with eternal motion in himself; and he connects this view with the affinity which he everywhere assumes to subsist between the human and divine spirit. With regard to the essence of the human soul he has no clear views; but he strenuously asserts its existence and phenomenal manifestation analogous to those of the Deity, and is disposed to ascribe to it immortality also. [74] Free Will he considers to be a truth of peculiar importance, probably from the practical consideration that on it responsibility and, therefore, morality itself ultimately rest.
From this brief abstract it will be seen that Cicero's speculative beliefs were to a great extent determined by his moral convictions, and by his strong persuasion of the dignity of human nature. This leads him to combat with vigour, and satirise with merciless wit, the Epicurean theory of life; and while his strong common sense forbids him to accept the Stoic doctrine in all its defiant harshness, he strengthens the Peripatetic view, to which he on the whole leans, by introducing elements drawn from it. The peculiar combination which he thus strives to form takes its colour from his own character and from the terms of his native language. The Greeks declare that the beautiful (_to kalon_) is good; Cicero declares that the honourable (_honestum_) alone is good. Where, therefore, the Greeks had spoken of _to kalon_, and we should speak of moral good, Cicero speaks of _honestum_, and founds precisely similar arguments upon it. This conception implies, besides self-regarding rectitude, the praise of others and the rewards of glory, and hence is eminently suited to the public-spirited men for whom he wrote. To it is opposed the base (_turpe_), that disgraceful evil which all good men would avoid. But as his whole moral theory is built on observation as much as on reading or reflection, he never stretches a rule too tight; he makes allowance for overpowering circumstances, for the temper and bent of the individual. Applicable to all who are engaged in an honourable career with the stimulus of success before them, his ethics were especially suited to the noble families of Rome to whom the approval of their conscience was indeed a necessity of happiness, but the approval of those whom they respected was at least equally so.