Was the Buddha serious about "not-self"?

German Buddhist monk Ven. Nyanatiloka (Buddhist Dictionary: Manual of Buddhist Terms and Doctrines), anatta entry edited by Dhr. Seven, CC Liu, Wisdom Quarterly

Anatta (Sanskrit an-atman) literally means "not-self," no-soul, egolessness, impersonality, selflessness.

It is the last -- and in some sense the most subtle and difficult to grasp -- of the Three Marks or Characteristics of Existence.

This teaching or doctrine of an-attā (not-self) states that neither within nor outside of the physical-and-mental phenomena of existence can there be found anything that -- in the ultimate sense -- can be regarded as a self, soul, self-existing ego-entity, or any other abiding essence.

 

This is the central doctrine of Buddhism. Without understanding it -- at least intellectually if not by penetrative insight [which destroys the delusion of compactness of self into its constituent parts, i.e., the Five Aggregates of Clinging], which would lead to enlightenment -- a real knowledge of Buddhism is altogether impossible.

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It is the only really specific Buddhist doctrine, the unique teaching of all buddhas in history, with which the entire structure of Buddhist Teaching stands or falls.

This body, this breath being here now

All the remaining Buddhist doctrines might, more or less, be found in other philosophical systems or religions.

But the anattā-doctrine is clearly and unreservedly taught only by buddhas, which is why the historical Buddha (Prince Siddhartha Gautama or Shakyamuni) was known as the Anattā-vādi, or "Teacher of Impersonality."

 

Whoever has not penetrated the impersonal nature of all existence, and does not comprehend that in reality there exists only this process of continual arising and passing of physical-and-mental phenomena, and that there is no independent ego-entity (separate from the impersonal parts or aggregates of existence that constitute it) within or without the process, that person will be unable to understand Buddhism, namely, the profound depth of the Buddhist teaching of the Four Noble (Ennobling/Enlightening) Truths... More