We are what we repeatedly do. Excellence then is not
an act, but a habit.
Aristotle
In the depth of winter, I finally learned that within me there lay
an invincible summer.
Albert Camus (1913-1960)
We make a living by what we get, we make a life by what we give .
Winston Churchill
The proof of the pudding is in the eating. By a small sample we may
judge of the whole piece.
Miguel de Cervantes, Don Quixote (1605-15)
For once you have tasted flight you will walk the earth with your eyes
turned skywards, for there you have been and there you will long to
return.
Leonardo da Vinci
Let him go where he will, he can only find so much beauty or worth
as he carries.
Variant: Though we travel the world over to find the beautiful,
we must carry it with us or we find it not.
Ralph Waldo Emerson
Whatever games are played with us, we must play no games with ourselves
but deal in our privacy with the last honesty and truth.
Ralph Waldo Emerson
To laugh often and much; To win the respect of intelligent people and the affection of children; To earn the approbation of honest critics and endure the betrayal of false friends; To appreciate beauty; To find the best in others; To give of one's self; To leave the world a bit better, whether by a healthy child, a garden patch, or a redeemed social condition; To have played and laughed with enthusiasm and sung with exultation; To know even one life has breathed easier because you have lived — This is to have succeeded.
Ralph Waldo Emerson
Whoever undertakes to set himself up as a judge of Truth and Knowledge
is shipwrecked by the laughter of the gods.
Albert Einstein (1879-1955)
The world is a dangerous place. Not because of the people who are evil;
but because of the people who don't do anything about it
Albert Einstein
If you have knowledge, let others light their candles at it.
Margaret Fuller
Believe those who are seeking the truth; doubt those who find it.
Andre Gide
I do not feel obliged to believe that the same God who has endowed
us with sense, reason, and intellect has intended us to forgo their
use.
Galileo Galilei
It contributes greatly towards a man's moral and intellectual health,
to be brought into habits of companionship with individuals unlike himself,
who care little for his pursuits, and whose sphere and abilities he
must go out of himself to appreciate.
Nathaniel Hawthorne
What lies behind us and what lies before us are tiny matters, compared
to what lies within us.
Oliver Wendell Holmes 
What is divine escapes men's notice because of their incredulity.
Heraclitus - Fragments
If there is anything more poignant than a body dying for lack of food
it is a mind dying for lack of light.
Victor Hugo - Les Misérables (1862) (IV.7.iv)
The deepest principle in human nature is the craving to be appreciated.
William James
The true measure of a man is how he treats someone who can do h im absoutely no good .
Samuel Johnson
In a mad world only the mad are sane.
Akira Kurosawa
People are like stained-glass windows. They sparkle and shine when
the sun is out, but when the darkness sets in, their true beauty is
revealed only if there is a light from within.
Elizabeth Kübler-Ross
I do not know what I may appear to the world; but to myself I seem
to have been only like a boy playing on the seashore, and diverting
myself in now and then finding a smoother pebble or a prettier shell
than ordinary, whilst the great ocean of truth lay all undiscovered
before me.
Isaac Newton
If I love a person, I would love that which most makes him a person:
the secrecy, the hiddenness, the solitude of his individual being, which
God alone can penetrate and understand.
Thomas Merton
You probably wouldn't worry about what people think of you if you could
know how seldom they do.
Olin Miller
The cause is hidden, but the result is known. [Lat., Causa latet: vis
est notissima.]
Ovid - Metamorphoses (IV, 287) 
What you leave behind is not what is engraved in stone monuments, but
what is woven into the lives of others.
Pericles
What an immense power over the life is the power of possessing distinct
aims. The voice, the dress, the look, the very motion of a person, define
and alter when he or she begins to live for a reason.
Elizabeth Stuart Phelps
My object is to show what I have found, not what I am looking for.
Pablo Picasso 
The mind is not a vessel to be filled but a fire to be kindled.
Plutarch
Love is space and time measured by the heart.
Marcel Proust
Just as the right to speak and the right to refrain from speaking are complementary components of a broader concept of individual freedom of mind, so also the individual’s freedom to choose his own creed is the counterpart of his right to refrain from accepting the creed established by the majority.
John Paul Stevens
It is not so much for its beauty that the forest makes a claim upon
men's hearts, as for that subtle something, that quality of air that
emanation from old trees, that so wonderfully changes and renews a weary
spirit.
Robert Louis Stevenson
Debt is the slavery of the free.
Pubilius Syrus
To sin by silence, when they should protest, makes cowards of men.
Ella Wheeler Wilcox:
Sex is the Tabasco sauce which an adolescent national palate sprinkles
on every course in the menu.
Mary Day Winn