Timothy Leary:  

Because that's the only kind of trip the Establishment would let it take.

Douglas Adams:  

Forty-two.

Nietzsche:  

Because if you gaze too long across the Road, the Road gazes also across you.

Plato:  

For the greater good.

Karl Marx:  

It was a historical inevitability.

Machiavelli: 

So that its subjects will view it with admiration, as a chicken which has the daring and courage to boldly cross the road, but also with fear, for whom among them has the strength to contend with such a paragon of avian virtue? In such a manner is the princely chicken's dominion maintained.

Albert Einstein:  

Whether the chicken crossed the road or the road crossed
the chicken depends upon your frame of reference.

Aristotle:  

To actualize its potential.

Buddha:  

If you ask this question, you deny your own chicken- nature.

Hippocrates:  

Because of an excess of light pink gooey stuff in its
pancreas.

Jacques Derrida:  

Any number of contending discourses may be discovered within the act of the chicken crossing the road, and each interpretation is equally valid as the authorial intent can never be discerned, because structuralism is DEAD, DAMMIT, DEAD!

Thomas de Torquemada:  

Give me ten minutes with the chicken and I'll find out.

Oliver North:  

National Security was at stake.

B.F. Skinner:  

Because the external influences which had pervaded its sensorium from birth had caused it to develop in such a fashion that it would tend to cross roads, even while believing these actions to be of its own free will.

Carl Jung:  

The confluence of events in the cultural gestalt necessitated that individual chickens cross roads at this historical juncture, and therefore
synchronicitously brought such occurrences into being.

Jean-Paul Sartre:  

In order to act in good faith and be true to itself, the chicken found it necessary to cross the road.

Ludwig Wittgenstein:  

The possibility of "crossing" was encoded into the objects "chicken" and "road", and circumstances came into being which caused the actualization of this potential occurrence.

Howard Cosell: 

It may very well have been one of the most astonishing events to grace the annals of history. An historic, unprecedented avian biped with the temerity to attempt such an Herculean achievement formerly relegated to Homo Sapien pedestrians is truly a remarkable occurrence.

Salvador Dali:  

The Fish.

Darwin:  

It was the logical next step after coming down from the trees.

Emily Dickinson:  

Because it could not stop for death.

Epicurus: 

For fun.

Ralph Waldo Emerson:  

It didn't cross the road; it transcended it.

Johann von Goethe:  

The eternal hen-principle made it do it.

Ernest Hemingway:  

To die. In the rain.

Werner Heisenberg:  

We are not sure which side of the road the chicken
was on, but it was moving very fast.

David Hume:  

Out of custom and habit.

Jack Nicholson:  

Cause it fucking wanted to. That's the fucking reason.

The Sphinx:  

You tell me.

Henry David Thoreau:  

To live deliberately ... and suck all the marrow out of life.

Mark Twain:  

The news of its crossing has been greatly exaggerated.

Wordsworth:  

To wander lonely as a cloud.

The Godfather:  

I didn't want its mother to see it like that.

Keats:  

Philosophy will clip a chicken's wings.

Blake:  

To see heaven in a wild fowl.

Othello:  

Jealousy.

Oscar Wilde:  

Why, indeed? One's social engagements whilst in town ought never expose one to such barbarous inconvenience - although, perhaps, if one must cross a road, one may do far worse than to cross it as the chicken in question.

Kafka: 

Hardly the most urgent enquiry to make of a low-grade insurance clerk who woke up that morning as a hen.

Macbeth:  

To have turned back were as tedious as to go o'er.

Freud: 

An die andere Seite zu kommen. (Much laughter).

Hamlet:  

That is not the question.

Pope:  

It was mimicking my Lord Hervey.