“No man is happy who does not think himself so.”

The Soul of Man Under Socialism (1891)

Chapters on Prayer

Source: The Liberty of Man, Woman and Child (1877)
Context: Only a few years ago there was a great awakening of the human mind. Men began to inquire by what right a crowned robber made them work for him? The man who asked this question was called a traitor. Others asked by what right does a robed hypocrite rule my thought? Such men were called infidels. The priest said, and the king said, where is this spirit of investigation to stop? They said then and they say now, that it is dangerous for man to be free. I deny it. Out on the intellectual sea there is room enough for every sail. In the intellectual air there is space enough for every wing.
The man who does not do his own thinking is a slave, and is a traitor to himself and to his fellow-men.

XI, 26, Parts of this passage has been heavily compared with later statements of René Descartes; in Latin and with a variant translations:
The City of God (early 400s)
Context: We both are, and know that we are, and delight in our being, and our knowledge of it. Moreover, in these three things no true-seeming illusion disturbs us; for we do not come into contact with these by some bodily sense, as we perceive the things outside of us of all which sensible objects it is the images resembling them, but not themselves which we perceive in the mind and hold in the memory, and which excite us to desire the objects. But, without any delusive representation of images or phantasms, I am most certain that I am, and that I know and delight in this. In respect of these truths, I am not at all afraid of the arguments of the Academicians, who say, What if you are deceived? For if I am deceived, I am. For he who is not, cannot be deceived; and if I am deceived, by this same token I am. And since I am if I am deceived, how am I deceived in believing that I am? for it is certain that I am if I am deceived. Since, therefore, I, the person deceived, should be, even if I were deceived, certainly I am not deceived in this knowledge that I am. And, consequently, neither am I deceived in knowing that I know. For, as I know that I am, so I know this also, that I know. And when I love these two things, I add to them a certain third thing, namely, my love, which is of equal moment. For neither am I deceived in this, that I love, since in those things which I love I am not deceived; though even if these were false, it would still be true that I loved false things. For how could I justly be blamed and prohibited from loving false things, if it were false that I loved them? But, since they are true and real, who doubts that when they are loved, the love of them is itself true and real? Further, as there is no one who does not wish to be happy, so there is no one who does not wish [themself] to be [into being]. For how can he be happy, if he is nothing?

Source: 1790s, The Age of Reason, Part I (1794)

Source: The Notebooks of Leonardo Da Vinci (1938), I Philosophy, p. 91
Context: He who suffers time to slip away and does not grow in virtue the more one thinks about him the sadder one becomes. No man has a capacity for virtue who sacrifices honour for gain. Fortune is powerless to help one who does not exert himself. That man becomes happy who follows Christ. There is no perfect gift without great suffering. Our triumphs and our pomps pass away; gluttony and sloth and enervating luxury have banished every virtue from the world; so that as it were wandering from its course our nature is subdued by habit. Now and henceforth it is meet that you cure yourself of laziness. The Master has said that sitting on down or lying under the quilts will not bring thee to fame. He who without it has frittered life away leaves no more trace of himself upon the earth than smoke does in the air or the foam on the water.

Source: The Virtue of Selfishness: A New Concept of Egoism