“It is in our own mind and not in exterior objects that we perceive most things; fools know scarcely anything because they are empty, and their heart is narrow; but great souls find in themselves a number of exterior things; they have no need to read or travel or to listen or to work to discover the highest truths; they have only to delve into themselves and search, if we may say so, their own thoughts.”

This Business of Living (1935-1950)

Un Art de Vivre (The Art of Living) (1939), The Art of Happiness

Aftermaths (p. 247). Note: Aftermaths was originally published as a standalone short story in 1986, but since then has usually been reprinted as a sort of appendix to Shards of Honor, which it follows naturally in the series arc.
Vorkosigan Saga, Shards of Honor (1986)

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?

[2012, Echoes of Perennial Wisdom, World Wisdom, 17, 978-1-93659700-0]
Spiritual path, Virtue

Source: Tuesdays with Morrie

Variant translation: When all your judgements are based on your own wisdom, you tend towards selfishness and fail by straying from the right path. Your own judgements are narrow minded and have no persuasive power or growth for others. It is best to consult a wise man when a fit decision does not occur to you. A wise man is a fair judge from an objective point of view. He is passing judgement for the benefit of others, not for his own sake. A judgement passed using only one's own wisdom is just like thrusting a stick into the ground and expecting it to grow!
Hagakure (c. 1716)
Context: It is difficult for a fool's habits to change to selflessness.... Because we do most things relying only on our own sagacity we become self-interested, turn our backs on reason, and things do not turn out well. As seen by other people this is sordid, weak, narrow and inefficient. When one is not capable of true intelligence, it is good to consult with someone of good sense. An advisor will fulfill the Way when he makes a decision by selfless and frank intelligence because he is not personally involved. This way of doing things will certainly be seen by others as being strongly rooted. It is, for example, like a large tree with many roots. One man's intelligence is like a tree that has been simply stuck in the ground.
We learn about the sayings and deeds of the men of old in order to entrust ourselves to their wisdom and prevent selfishness. When we throw off our own bias, follow the sayings of the ancients, and confer with other people, matters should go well and without mishap.

31 October 1966; p. 59
1960's, Conversations with Samuel Beckett and Bram van Velde' (1965 - 1969)

Source: The Society of Mind (1987), Ch.1