Agnostics, unlike atheists, do not deny the existence of God, but instead, believe that no substantial evidence has been found to either prove or disprove the existence of God.
Agnosticism first appeared in the time of the pre-Socratic philosophers, in the fourth century BCE. These philosophers believed that nothing could be known with absolute certainty, especially questions pertaining to God and the supernatural.
The term agnosticism was invented by Thomas Huxley around 1869. Huxley did not believe in the Judeo-Christian idea of God, but neither did he deny its existence. He felt that no substantial evidence existed to support the existence or absence of a God or supernatural power.
Agnostics usually hold the question of God’s existence open, until evidence suggests otherwise. They are willing to change their belief if some solid evidence or logical proof is found in the future. Many, however, have taken the position that there is no logical way in which the existence or non-existence of God can be proven.
Within the agnostic philosophy, there are several categories: agnostic theists, who believe that a deity probably exists; agnostic atheists, who believe that it is highly improbable that a deity exists; empirical agnostics, who believe that God may exist, but that little or nothing can be known about it, and agnostic humanists, who are also undecided about the existence of God but do not feel the question is particularly important.
Like atheism, agnosticism enjoyed its greatest popularity in the nineteenth century. Today, the exact number of agnostics is unknown, as many polls group atheists, agnostics, nonreligious, and skeptics together.