There are two types of men, those who are afraid to lose God, and those who are afraid that they might find HimÖ
--Blaise Pascal, philosopher and scientist

Does God exist?

This is surely a fundamental question that nearly all humans have pondered with throughout human history. The vast array of religions are a testimony to the human tendency to grasp at the divine. This in itself is perhaps the strongest testimony to Godís existence. It can be said that all humans have an innate desire; an emptiness that they feel must be filled. The human quest for power, riches, sensual pleasure, security, fame and indulgence in natural pleasures is a response to the heartfelt desire for a higher goodness. Temporal pleasures and even natural love is often transitory and ultimately unfulfilling. As humans indulge in their passions their desires continue to go unfulfilled. Many attempt to fill the void with increasing worldly pleasures with little results.

Such powerful and elusive desires are a cry from the soul which seeks something that can not be gratified by the things of this world. For the moment we will consider discontent of the heart as a mark of God calling us to embrace him.

But I demand physical proof!

St. Thomas Aquinas proposed five proofs in which humans can use natural reason to prove the existence of God through extrinsic evidence. Through the use of natural reason we can logically conclude in the existence of God. Yet strictly speaking, Godís existence cannot be definitively proven through laboratory tests and experimental science.

Not all things are subject to experimental science. It is illogical to say, "If I can not see, taste, touch, feel or hear something it must not exist!" Reason and extrinsic evidence must also be considered.

Experimental science and intrinsic evidence cannot definitively prove historical events, and yet by reason we know they have occurred. And surely were science falters and extrinsic evidence fail, reason and intrinsic evidence can prove the spiritual which can not be measured by material sciences.

St. Thomas Aquinas five proofs of the existence of God

Aquinasí first proof is through the argument of motion. It can be noted that some things in the universe are in motion and it follows that whatever is in the state of motion must have been placed in motion by another such act. Motion in itself is nothing less then the reduction of something from the state of potentiality to actuality. Because something can not be in potentiality and actuality simultaneously, it follows that something can not be a mover of itself.

A simple example of this is a rubber ball motionless on a flat surface. It has the potential for motion, but is not currently in the state of actual motion. In order for this to happen, something else in motion must set the ball in motion, be that gravity, another moving object or the wind.

And yet something must have set that object in motion as well (even gravity, a force caused by matter warping the space-time fabric, attributes its existence to pre-existing matter and the exchange of pre-existing graviton particles). Thus pre-existing motions cause all motions.

Yet, this chain can not extend into infinity because that would deny a first mover that set all else in motion. Without a first mover, nothing could be set in motion. Thus we acknowledge the first and primary mover as God.
http://www.saintaquinas.com/belief_in_God.html