This section is for Leonardo's most popular and significant quotes. 

There are three classes of people: those who see. Those who see when they are shown. Those who do not see.

"O MARVELOUS, O STUPENDOUS NECESSITY, THOU WITH SUPREME REASON COMPELLEST ALL EFFECTS TO BE THE DIRECT RESULT OF THEIR CAUSES; AND BY A SUPREME AND IRREVOCABLE LAW EVERY NATURAL ACTION OBEYS THEE BY THE SHORTEST POSSIBLE PROCESS. WHO WOULD BELIEVE THAT SO SMALL A SPACE COULD CONTAIN THE IMAGES OF ALL THE UNIVERSE? O MIGHTY PROCESS! WHAT TALENT CAN AVAIL TO PENETRATE A NATURE SUCH AS THESE? WHAT TONGUE WILL IT BE THAT CAN UNFOLD SO GREAT A WONDER? VERILY NONE! THIS IT IS THAT GUIDES THE HUMAN DISCOURSE TO THE CONSIDERING OF DIVINE THINGS. HERE THE FORMS, HERE THE COLOURS, HERE ALL THE IMAGES OF EVERY PART OF THE UNIVERSE ARE CONTRACTED TO A POINT. WHAT POINT IS SO MARVELOUS? O WONDERFUL, O STUPENDOUS NECESSITY---BY THY LAW THOU CONSTRAINEST EVERY EFFECT TO BE THE DIRECT RESULT OF ITS CAUSE BY THE SHORTEST PATH.

THESE ARE MIRACLES...FORMS ALREADY LOST, MINGLED TOGETHER IN SO SMALL A SPACE IT CAN RECREATE AND RECOMPENSE BY EXPANSION. DESCRIBE IN THY ANATOMY WHAT PROPORTION THERE IS BETWEEN DIAMETERS OF ALL THE LENSES IN THE EYE AND THE DISTANCE FROM THESE TO THE CRYSTALLINE LENS."

I have always felt it is my destiny to build a machine that would allow man to fly

Iron rusts from disuse; stagnant water loses its purity and in cold weather becomes frozen; even so does inaction sap the vigor of the mind.

Do not reveal, if liberty is precious to you; my face is the prison of love.

Be not false about the past.

The greatest deception men suffer is from their own opinions.

Many have made a trade of delusions and false miracles, deceiving the stupid multitude. Pharisees — that is to say, friars.

He who walks straight rarely falls.

Reserve the great matters till the end, and the small matters give at the beginning.

Threats alone are the weapons of the threatened man.

Having wandered some distance among gloomy rocks, I came to the entrance of a great cavern…Two contrary emotions arose in me, fear and desire—fear of the threatening dark cavern, desire to see whether there were any marvelous things in it. 

Patience serves us against insults precisely as clothes do against the cold. For if you multiply your garments as the cold increases, that cold cannot hurt you; in the same way increase your patience under great offences, and they cannot hurt your feelings.

The  water you touch in a river is the last of that which has passed, and the first of that which is coming. Thus it is with time present.

We are deceived by promises and time disappoints us...
 

Obstacles cannot crush me. Every obstacle yields to stern resolve. He who is fixed to a star does not change his mind.

As a well-spent day brings happy sleep, so life well used brings happy death.

He who thinks little, errs much.

Such as harm is when it hurts me not, is good which avails me not.

Ask counsel of him who rules himself well.

He who does not punish evil commands it to be done.

The grave will fall in upon him who digs it.

You can have no dominion greater or less than that over yourself.

He who offends others, does not secure himself.

Life well spent is long.

The Medici created and destroyed me.

Just as eating against one’s will is injurious to health, so studying without a liking for it spoils the memory, and it retains nothing it takes in.[

Movement will cease before we are weary of being useful.

Poor is the pupil that does not surpass his master.

He who possesses most must be most afraid of loss.

He who wishes to be rich in a day will be hanged in a year.

That man is of supreme folly who always wants for fear of wanting; and his life flies away while he is still hoping to enjoy the good things which he has with extreme labour acquired.

The senses are of the earth; Reason, stands apart in contemplation.

Necessity is the mistress and guide of nature.

Tell me if anything was ever done.

What is fair in men, passes away, but not so in art.

Our body is dependent on heaven and heaven on the Spirit.

To lie is so vile, that even if it were in speaking well of godly things it would take off something from God's grace; and Truth is so excellent, that if it praises but small things they become noble.

The motive power is the cause of all life.

This writing distinctly about the kite seems to be my destiny, because among the first recollections of my infancy, it seemed to me that, as I was in my cradle, a kite came to me and opened my mouth with its tail, and struck me several times with its tail inside my lips.

Nothing is so much to be feared as Evil Report.

It is easier to resist at the beginning than at the end.

Every quantity is intellectually conceivable as infinitely divisible.

Oh! human stupidity, do you not perceive that, though you have been with yourself all your life, you are not yet aware of the thing you possess most of, that is of your folly? and then, with the crowd of sophists, you deceive yourselves and others, despising the mathematical sciences, in which truth dwells and the knowledge of the things included in them. And then you occupy yourself with miracles, and write that you possess information of those things of which the human mind is incapable and which cannot be proved by any instance from nature. And you fancy you have wrought miracles when you spoil a work of some speculative mind, and do not perceive that you are falling into the same error as that of a man who strips a tree of the ornament of its branches covered with leaves mingled with the scented blossoms or fruit.

Men are in error when they lament the flight of time, accusing it of being too swift, and not perceiving that it is sufficient as it passes; but good memory, with which nature has endowed us, causes things long past to seem present.

Our life is made by the death of others.

The painter strives and competes with nature.

The image of the sun where it falls appears as a thing which covers the person who attempts to cover it.

Happy will they be who lend ear to the words of the Dead.

Constancy does not begin, but is that which perseveres.

We, by our arts may be called the grandsons of God.

It vexes me greatly that having to earn my living has forced me to interrupt the work and to attend to small matters.

Science is the captain, and practice the soldiers.

Every instrument requires to be made by experience.

You do ill if you praise, and still worse if you reprove in a matter you do not understand.

The sun gives spirit and life to plants and the earth nourishes them with moisture.

Necessity is the mistress and guardian of Nature.

The atmosphere is blue by reason of the darkness above it because black and white make blue.

All our knowledge has its origin in our perceptions.

Human subtlety...will never devise an invention more beautiful, more simple or more direct than does nature, because in her inventions nothing is lacking, and nothing is superfluous.

Nature is full of infinite causes that have never occurred in experience.

Experience never errs; it is only your judgments that err by promising themselves effects such as are not caused by your experiments.

Mechanics is the paradise of the mathematical sciences because by means of it one comes to the fruits of mathematics.

Shun those studies in which the work that results dies with the worker.

"Science is the observation of things possible, whether present or past. Prescience is the knowledge of things which may come to pass, though but slowly." 

Any one who in discussion relies upon authority uses, not his understanding, but rather his memory.

Those who are in love with practice without knowledge are like the sailor who gets into a ship without rudder or compass and who never can be certain whether he is going. Practice must always be founded on sound theory, and to this Perspective is the guide and the gateway; and without this nothing can be done well in the matter of drawing.

The acquisition of any knowledge is always of use to the intellect, because it may thus drive out useless things and retain the good. For nothing can be loved or hated unless it is first known.

Men born in hot countries love the night because it refreshes them and have a horror of light because it burns them; and therefore they are of the colour of night, that is black. And in cold countries it is just the contrary.

Drawing is based upon perspective, which is nothing else than a thorough knowledge of the function of the eye. 

The boundaries of bodies are the least of all things. 

The instant the atmosphere is illuminated it will be filled with an infinite number of images which are produced by the various bodies and colours assembled in it. And the eye is the target, a lodestone, of these images. 
 

The earth is not in the centre of the Sun's orbit nor at the centre of the universe, but in the centre of its companion elements, and united with them. And any one standing on the moon, when it and the sun are both beneath us, would see this our earth and the element of water upon it just as we see the moon, and the earth would light it as it lights us.

King of the animals — as thou hast described him — I should rather say king of the beasts, thou being the greatest — because thou hast spared slaying them, in order that they may give thee their children for the benefit of the gullet, of which thou hast attempted to make a sepulchre for all animals; and I would say still more, if it were allowed me to speak the entire truth . But we do not go outside human matters in telling of one supreme wickedness, which does not happen among the animals of the earth, inasmuch as among them are found none who eat their own kind, unless through want of sense.

Though human ingenuity may make various inventions which, by the help of various machines answering the same end, it will never devise any inventions more beautiful, nor more simple, nor more to the purpose than Nature does; because in her inventions nothing is wanting, and nothing is superfluous, and she needs no counterpoise when she makes limbs proper for motion in the bodies of animals. But she puts into them the soul of the body, which forms them that is the soul of the mother which first constructs in the womb the form of the man and in due time awakens the soul that is to inhabit it.

Now you see that the hope and the desire of returning home and to one's former state is like the moth to the light, and that the man who with constant longing awaits with joy each new spring time, each new summer, each new month and new year — deeming that the things he longs for are ever too late in coming — does not perceive that he is longing for his own destruction. But this desire is the very quintessence, the spirit of the elements, which finding itself imprisoned with the soul is ever longing to return from the human body to its giver. And you must know that this same longing is that quintessence, inseparable from nature, and that man is the image of the world.

Amid the vastness of the things among which we live, the existence of nothingness holds the first place; its function extends over all things that have no existence, and its essence, as regards time, lies precisely between the past and the future, and has nothing in the present. This nothingness has the part equal to the whole, and the whole to the part, the divisible to the indivisible; and the product of the sum is the same whether we divide or multiply, and in addition as in subtraction; as is proved by arithmeticians by their tenth figure which represents zero; and its power has not extension among the things of Nature.

O sleepers! what a thing is slumber! Sleep resembles death. Ah, why then dost thou not work in such wise as that after death thou mayst retain a resemblance to perfect life, when, during life, thou art in sleep so like to the hapless dead?

Nothing is that which fills no space. If one single point placed in a circle may be the starting point of an infinite number of lines, and the termination of an infinite number of lines, there must be an infinite number of points separable from this point, and these when reunited become one again; whence it follows that the part may be equal to the whole.
 

"Study me, reader, if you delight in me, because on very few occasions shall I return to the world, and because the patience for this profession is found in very few, and only in those who wish to compose things anew. Come, oh men, to see the miracles that such studies will disclose to nature." 

The part always has a tendency to reunite with its whole in order to escape from its imperfection.

O Man, who will discern in this work of mine the wonderful works of Nature, if you think it would be a criminal thing to destroy it, reflect how much more criminal it is to take the life of a man; and if this, his external form, appears to thee marvellously constructed, remember that it is nothing as compared with the soul that dwells in that structure; for that indeed, be it what it may, is a thing divine. Leave it then to dwell in His work at His good will and pleasure, and let not your rage or malice destroy a life — for indeed, he who does not value it, does not himself deserve it.

Every body in light and shade fills the surrounding air with infinite images of itself; and these, by infinite pyramids diffused in the air, represent this body throughout space and on every side.
 

"Why does the eye see a thing more clearly in dreams than the imagination when awake?" 

I myself have proved it to be of no small use, when in bed in the dark, to recall in fancy the external details of forms previously studied, or other noteworthy things conceived by subtle speculation; and this is certainly an admirable exercise, and useful for impressing things on the memory.

That is not riches, which may be lost; virtue is our true good and the true reward of its possessor. That cannot be lost; that never deserts us, but when life leaves us. As to property and external riches, hold them with trembling; they often leave their possessor in contempt, and mocked at for having lost them.

If you meet with any one who is virtuous do not drive him from you; do him honour, so that he may not have to flee from you and be reduced to hiding in hermitages, or caves or other solitary places to escape from your treachery; if there is such an one among you do him honour, for these are our Saints upon earth; these are they who deserve statues from us, and images...

Blind ignorance misleads us thus and delights with the results of lascivious joys. Because it does not know the true light. Because it does not know what is the true light. Vain splendour takes from us the power of being .... behold! for its vain splendour we go into the fire, thus blind ignorance does mislead us. That is, blind ignorance so misleads us that... O! wretched mortals, open your eyes.