“[P]hilosophy is not a matter for adjustment, as a means, to national requirements, but every nation and individual that cultivates philosophy should aim at having a true philosophy.”

As quoted in The Works of the Emperor Julian (1923) by Wilmer Cave France Wright, p. 39; also in The Missing Jesus: Rabbinic Judaism and the New Testament (2003) by Craig Alan Evans, Carl A. Elliott, Bruce Chilton, Jacob Neusner
General sources

Optimism (1903)

Variant translation: Philosophy is not a theory but an activity. A philosophical work consists essentially of elucidations. The result of philosophy is not a number of "philosophical propositions." but to make propositions clear.
Original German: Der Zweck der Philosophie ist die logische Klärung der Gedanken. Die Philosophie ist keine Lehre, sondern eine Tätigkeit. Ein philosophisches Werk besteht wesentlich aus Erläuterungen. Das Resultat der Philosophie sind nicht „philosophische Sätze“, sondern das Klarwerden von Sätzen. Die Philosophie soll die Gedanken, die sonst, gleichsam, trübe und verschwommen sind, klar machen und scharf abgrenzen.
1920s, Tractatus Logico-Philosophicus (1922)
Context: Philosophy aims at the logical clarification of thoughts. Philosophy is not a body of doctrine but an activity. A philosophical work consists essentially of elucidations. Philosophy does not result in 'philosophical propositions', but rather in the clarification of propositions. Without philosophy thoughts are, as it were, cloudy and indistinct: its task is to make them clear and to give them sharp boundaries. (4.112)

The Source and Aim of Human Progress (1919)

On Lord Bacon (1837)

Source: Rhetoric as Philosophy (1980), p. 40

Source: 1910s, Mysticism and Logic and Other Essays http://archive.org/stream/mysticism00russuoft/mysticism00russuoft_djvu.txt (1918), Ch. 1: Mysticism and Logic

Letter 74 (76) to Albert Burgh (1675) http://oll.libertyfund.org/?option=com_staticxt&staticfile=show.php%3Ftitle=1711&chapter=144250&layout=html&Itemid=27
Context: You seem to wish to employ reason, and ask me, "How I know that my philosophy is the best among all that have ever been taught in the world, or are being taught, or ever will be taught?" a question which I might with much greater right ask you; for I do not presume that I have found the best philosophy, I know that I understand the true philosophy. If you ask in what way I know it, I answer: In the same way as you know that the three angles of a triangle are equal to two right angles: that this is sufficient, will be denied by no one whose brain is sound, and who does not go dreaming of evil spirits inspiring us with false ideas like the true. For the truth is the index of itself and of what is false.
But you, who presume that you have at last found the best religion, or rather the best men, on whom you have pinned your credulity, you, "who know that they are the best among all who have taught, do now teach, or shall in future teach other religions. Have you examined all religions, ancient as well as modern, taught here and in India and everywhere throughout the world? And, if you, have duly examined them, how do you know that you have chosen the best" since you can give no reason for the faith that is in you? But you will say, that you acquiesce in the inward testimony of the Spirit of God, while the rest of mankind are ensnared and deceived by the prince of evil spirits. But all those outside the pale of the Romish Church can with equal right proclaim of their own creed what you proclaim of yours.
As to what you add of the common consent of myriads of men and the uninterrupted ecclesiastical succession, this is the very catch-word of the Pharisees. They with no less confidence than the devotees of Rome bring forward their myriad witnesses, who as pertinaciously as the Roman witnesses repeat what they have heard, as though it were their personal experience. Further, they carry back their line to Adam. They boast with equal arrogance, that their Church has continued to this day unmoved and unimpaired in spite of the hatred of Christians and heathen. They more than any other sect are supported by antiquity. They exclaim with one voice, that they have received their traditions from God himself, and that they alone preserve the word of God, both written and unwritten. That all heresies have issued from them, and that they have remained constant through thousands of years under no constraint of temporal dominion, but by the sole efficacy of their superstition, no one can deny. The miracles they tell of would tire a thousand tongues. But their chief boast is that they count a far greater number of martyrs than any other nation, a number which is daily increased by those who suffer with singular constancy for the faith they profess; nor is their boasting false. I myself knew among others of a certain Judah called the faithful, who in the midst of the flames, when he was already thought to be dead, lifted his voice to sing the hymn beginning, "To thee, o God, I offer up my soul", and so singing perished.

Source: Rhetoric as Philosophy (1980), pp. 31-32
Context: In the second part of the Phaedrus Plato attempts to clarify the nature of “true” rhetoric. … it does not arise from a posterior unity which presupposes the duality of ratio and passio, but illuminates and influences the passions through its original, imaginative characters. Thus philosophy is not a posterior synthesis of pathos and logos but the original unity of the two under the power of the original archai. Plato sees true rhetoric as psychology which can fulfill its truly “moving” function only if it masters original images [eide]. Thus the true philosophy is rhetoric, and the true rhetoric is philosophy, a philosophy which does not need an “external” rhetoric to convince, and a rhetoric that does not need an “external” content of verity.