The least important things, sometimes, my dear boy, lead to the greatest discoveries.
The Second Doctor:
Well now I know you're mad, I just wanted to make sure.
The Third Doctor:
Courage isn't just a matter of not being frightened, you know. It's being afraid and doing what you have to do anyway.
The Fourth Doctor:
You're a classic example of the inverse ratio between the size of the mouth and the size of the brain.
The Fifth Doctor:
An apple a day keeps the... Ah, never mind.
The Sixth Doctor:
Planets come and go. Stars perish. Matter disperses, coalesces, forms into other patterns, other worlds. Nothing can be eternal.
The Seventh Doctor:
Yes, that's right, you're going. You've been gone for ages. You're already gone. You're still here. You've just arrived. I haven't even met you yet. It all depends on who you are and how you look at it. Strange business, time.
The Eighth Doctor:
I love humans. Always seeing patterns in things that aren't there.
The Ninth Doctor:
The thing is, Adam, time travel is like visiting Paris. You can't just read the guidebook, you've got to throw yourself in! Eat the food, use the wrong verbs, get charged double and end up kissing complete strangers!
The Tenth Doctor:
People assume that time is a strict progression of cause to effect... but actually, from a non-linear, non-subjective viewpoint, it's more like a big ball of wibbly-wobbly... timey-wimey... stuff.
The Eleventh Doctor:
The way I see it, every life is a pile of good things and bad things. The good things don’t always soften the bad things, but vice versa the bad things don’t always spoil the good things and make them unimportant.