Agnostic atheism is the philosophy that encompasses both atheism and agnosticism. Due to definitional variance, an agnostic atheist does not believe in God or gods and by extension holds true one or more of these statements:
1) The existence and nonexistence of deities is currently unknown and perhaps unknowable.
2) Knowledge of the existence and nonexistence of deities is irrelevant or unimportant.[citation needed]
3) Abstention from claims of knowledge of the existence and nonexistence of deities is optimal.
4) While the concepts of atheism and agnosticism occasionally overlap, they are distinct because atheism is generally defined as a condition of being without theistic beliefs while agnosticism is usually defined as an absence of knowledge (or any claim of knowledge); therefore, an agnostic person may also identify as an atheist, a theist, or one who endorses neither position.
One of the earliest explanations of agnostic atheism is that of Robert Flint, in his Croall Lecture of 1887-1888 (published in 1903 under the title Agnosticism):
The atheist may however be, and not unfrequently is, an agnostic. There is an agnostic atheism or atheistic agnosticism, and the combination of atheism with agnosticism which may be so named is not an uncommon one. (p.49)
If a man has failed to find any good reason for believing that there is a God, it is perfectly natural and rational that he should not believe that there is a God; and if so, he is an atheist... if he go farther, and, after an investigation into the nature and reach of human knowledge, ending in the conclusion that the existence of God is incapable of proof, cease to believe in it on the ground that he cannot know it to be true, he is an agnostic and also an atheist - an agnostic-atheist - an atheist because an agnostic... while, then, it is erroneous to identify agnosticism and atheism, it is equally erroneous so to separate them as if the one were exclusive of the other... (p.50-51)