Some scholars contend that if the Buddha had been marketing ātman as the cardinal doctrine of Buddhism then why, they ask, was its light hid under a bushel unlike with the ātman of Upaniṣads? The single most important reason ātman was not stressed as much in Buddhism is the Buddha taught by way of what is not ātman, for example, the five skandhas (the psychophysical body) are not ātman inasmuch as they are imperfect being synonymous with suffering and impermanence. More practical, the Buddha's teaching assumed ātman—but ātman as yet unrealized by those to whom he taught because of it being veiled by the psychophysical body.
Rejecting what is not my ātman is much easier and more expedient than imagining what this mysterious ātman might be in the example of the Upaniṣads, and getting it wrong; having constructed a false ātman with the imagination. Speaking of the samaṇas and the brāmaṇas during the time of the Buddha, the Buddha said they envisage ātman as one or more of the five grasping skandhas (S. iii. 46) which make up the psychophysical body. Clearly, the Buddha did not see it this way. His awakening saw the ātman as spiritually distinct from conditioned reality including the psychophysical body. Neither was ātman asti as something extended or existent nor was it nasti as not anything at all. It was unique in this respect.
The effectiveness of negation, that is, the Buddha teaching us by way of what is not our ātman, is because the unreal can be negated but never the real; so that what remains at the end of this process, is ultimately real. This, also, is the truth of the via negativa. If there was no non-negatable transcendent ground (ātman) to begin with, it would not be possible to say of each and every skandha, this is not my ātman.
On the other hand, those who contend there is no ātman do so, mistakenly, from the standpoint of the five skandhas. For example, taking the position of the first skandha, which is physical form, they see no ātman and so it is with the rest of the skandhas up to consciousness which they see as being bereft of ātman. This they believe is solid proof that there is no ātman! But this was not the position of the Buddha. Rather it is the position of the nihilist who adheres to nāsti ātman (lit. there is no ātman). Ultimately it is by the very self (praty-ātman) that nirvana is realized. Lacking ātman how is attainment of nirvana even possible?