Ajñāna (Sanskrit: अज्ञान, (Vedic) IPA: /ɐd͡ʑ.ɲɑː.nɐ/; (Classical) IPA: /ɐd͡ʑˈɲɑː.n̪ɐ/) is an ancient nāstika, or 'heterodox,' Indian school of agnosticism that embraces radical skepticism. It emerged as a Śramaṇa movement and was a major rival of early Buddhism, Jainism and the Ājīvika school. They have been recorded in Buddhist and Jain texts. They held that it was impossible to obtain knowledge of metaphysical nature or ascertain the truth value of philosophical propositions; and even if knowledge was possible, it was useless and disadvantageous for final salvation. They specialized in refutation without propagating any positive doctrine of their own. Sanjaya Belatthiputta was one of the major proponents of this school of thought.

All of our knowledge of the Ajñāna come from the Buddhists and Jain sources. The Ajñāna view points are recorded in Theravada Buddhism's Pāli Canon in the Brahmajala Sutta and Samaññaphala Sutta and in the Sūyagaḍaṃga of Jainism. Along with these texts, the sayings and opinions of the Sceptics (ajñānikāḥ, ajñānināḥ) has been preserved by Jain writer Silanka, from the ninth century, commenting on the Sutrakritanga. Silanka considers sceptics "those who claim that scepticism is best" or as "those in whom no knowledge, i.e. scepticism, is evident". Apart from the specific technical meaning, Silanka also uses the word ajñānikah in a more general sense to mean anyone who is ignorant.

The traces of scepticism can be found in Vedic sources such as in the Nasadiya hymn and hymn to sraddha (faith) in Rigveda. In Brahmanas and Early Upanishads doubt regarding a person's existence after death is cast, while the Yajñavalkya argued for the impossibility of knowing the ultimate reality or the atman. However the flourishing of sceptical thoughts seems to have occurred in a period with diverse, conflicting, and irreconcilable theories, regarding morality, metaphysics, and religious beliefs. It is natural, in the absence of a commonly accepted criterion of truth, for some people to wonder if any theory could be true at all. The Sceptics specifically pointed to the conflicting theories of atman and the requirement of omniscience, and hence the criticism of omniscience, to obtain true knowledge. A proliferation of view points existed during the period immediately preceding the rise of Buddhism, as attested in the Buddhist and Jain texts. The Buddhist Brahmajal Sutta lists four types (or schools) of Sceptics along with fifty-eight other schools of thought, while the Jain Sutrakrtanga lists sixty-seven "schools" of Sceptics among three hundred and sixty-three different schools of thought. While the list is artificially constructed according to Jain categories, the four main schools of thought, Kriyavada, Akriyavada, Ajnanikavada, and Vainayikavada, and their subgroups must have existed. Thus, philosophical Scepticism is thought to have appeared around this historic time frame.

The Ajñāna claimed that the possibility of knowledge is doubtful since the claims to knowledge were mutually contradictory. Silanka quotes, "They posit the theory that since those who claim knowledge make mutually contradictory assertions, they cannot be stating the truth." Regarding Sceptic's point of view, Silanka in his commentary writes, as translated by Jayatilleke:

For they (i.e. the Sceptics) say that those who claim knowledge (jñaninah) cannot be stating actual facts since their statements are mutually contradictory, for even with regard to the category of the soul, some assert that the soul is omnipresent (sarvagatam) and other that it is not omnipresent (asarvagatam), some (say) it is of the size of a digit (angustaparvamatram) others that it is of the size of a kernel of a grain of millet (syamakatandulamatram) some say it both has form and is formless (murtamamurtam), some that it resides in the heart (hrdayamadhyavartinam) and (others) that it is located in the forehead (lalatavyavasthitam), etc. -- in respect of every category there is no uniformity in their assertion.

The conflicting theories of atman can be traced to Early Upanisads. The idea of atman "made of everything" (sarvamayah, idammayah adomayah) would be omnipresent (sarvagatam) (Brhadaranyaka 4.4.5) while the transcendent atman defined negatively (Brhadaranyaka 3.9.26) would not be so. Again at Katha 2.3.17 the atman is of "the size of a digit", while at Chandogya 3.14.3, the atman is "smaller than a kernel of a grain of millet". Again at Brhadaranyaka 2.3.1, Brahman which is identical with the atman is said "both to have form and also be formless." Likewise at Katha 2.3.17 the atman "resides in the heart" while at Aitareya Aranyaka 2.1.4.6 it is located in the head.

In the same passage, Silanka continues:

There is no one with an outstanding intellect whose statements may be regarded as authoritative; even if such a person existed, he cannot be discovered by one with a limited vision according to the maxim that "one who is not omniscient does not know everything" for it is said "how can one desiring to know that a certain person is omniscient at a certain time do so if he is devoid of that person's intellect, his knowledge and his consciousness"; owing to the absence of the knowledge of the means, it cannot properly be accomplished; it cannot be accomplished because of the mutual dependence (of the two); for it is said "without a super-knowledge (visistaparijñana) the knowledge of the means is not attained and as a result there is no attainment of the super-knowledge of the object".