Samuel Johnson, 18th century British poet, essayist, and literary critic:

“The chains of habit are too weak to be felt until they are too strong to be broken.”

Dr. Frank Crane, Presbyterian minister, speaker, and columnist:

Habits are safer than rules; you don’t have to watch them. And you don’t have to keep them, either. They keep you.

Fyodor Dostoevsky, Russian writer and existentialist philosopher:

“The second half of a man’s life is made up of nothing but the habits he has acquired during the first half.”

Confucius, 5th century Chinese teacher and social philosopher:

“Men’s natures are alike; it is their habits that separate them.”

Georges Gurdjieff, Greek-Armenian mystic and spiritual teacher of sacred dance:

“Every grown-up man consists wholly of habits, although he is often unaware of it and even denies having any habits at all.”

Unknown:

“A habit is something you can do without thinking – which is why most of us have so many of them.”

Jim Ryun, American politician and former track athlete:

“Motivation is what gets you started. Habit is what keeps you going.”

Yiddish Proverb:

“Bad habits are easier to abandon today than tomorrow.”

Aristotle, seminal Greek naturalist and rationalist and father of Western philosophy. (Sidenote: I studied Aristotle extensively during my undergrad, and completed papers on Aristotle’s mathematics [pdf] and concept of justice.)

“We are what we repeatedly do. Excellence then, is not an act, but a habit.”