5. As to the Cyrenaic doctrine of proofs, no evidence remains.
In many of these opinions we recognize the happy, careless, selfish disposition which characterized their author; and the system resembles in most points those of Heracleitus and Protagoras, as given in Plato's Theaetetus.
The doctrines that a subject only knows objects through the prism of the impression which he receives, and that man is the measure of all things, are stated or implied in the Cyrenaic system, and lead at once to the consequence, that what we call reality is appearance; so that the whole fabric of human knowledge becomes a fantastic picture.
The principle on which all this rests, viz. that knowledge is sensation, is the foundation of Locke's modern ideology, though he did not perceive its connexion with the consequences to which it led the Cyrenaics. To revive these was reserved for Hume.
The ancient authorities on this subject are Diogenes Laertius, 2.65, &c.; Sextus Empiricus, ad v. Math.
7.11; the places in Xenophon and Aristotle already referred to; Cic. Tusc.
3.13, 22, Acad.
4.7, 46; Euseb. Praep. Evang.
The chief modern works are, Kunhardt, Dissertatio philos.-historica de Aristippi Philosophia morali,
Helmstädt, 1795, 4to.; Wieland, Aristipp und Einige seiner Zeitgenossen,
Leipz., 1800-1802 ; Ritter, Geschichte der Philosophie,
7.3; Brucker, Historia Critica Philosophiae,