An-attā, Not Self
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The following definition of this vital term is taken from the standard 'Buddhist Dictionary' of Bhikkhu Nyanatiloka. 'Anatta: "Non-Ego, Not-Self", i.e. the fact that neither within these bodily and mental phenomena of existence, nor outside them, can be found anything that in the ultimate sense could be regarded as a self-reliant real Ego-entity, or Personality. This is the central doctrine of Buddhism without an understanding of which a real knowledge of Buddhism is altogether impossible ... the Buddha was known as the Anatta-vadi, or teacher of Impersonality:

We see, therefore, that the meaning and significance of anatta is all-important. Thought of as 'humility', it is the basis of individual Buddhist altruism, and the man who treads properly the Middle Path of the Compassionate One becomes aware that 'humility is the greatest penance' (Dp vs. 184), the greatest character-builder. 'Self is the lord of self, who else could be the lord? With self well subdued, a man finds a lord such as few can find' (vs. 160).