Second, they developed theories on what "Atman is and is not". As proofs for the proposition "self/soul exists", for example, Nyaya scholars argued that personal recollections and memories of the form "I did this so many years ago" implicitly presume that there is a self that is substantial, continuing, unchanged, and existent. Nyayasutra, a 2nd-century CE foundational text of Nyaya school of Hinduism, states that the soul is a proper object of human knowledge. It also states that soul is a real substance that can be inferred from certain signs, objectively perceivable attributes. For example, in book 1, chapter 1, verses 9 and 10, Nyayasutra states Book 2, chapter 1, verses 1 to 23, of the Nyayasutras posits that the sensory act of looking is different from perception and cognition–that perception and knowledge arise from the seekings and actions of "Ātman" (soul). The Naiyayikas emphasize that the "Ātman" has qualities, but is different from its qualities. For example, desire is one of many qualities of the "Ātman", but the "Ātman" does not always have desire, and in the state of liberation, for instance, the "Ātman" is without desire. The concept of "Ātman" in Samkhya, the oldest school of Hinduism, is quite similar to one in Advaita Vedanta school. Both Samkhya and Advaita consider the ego ("asmita", "ahamkara") rather than the "Ātman" to be the cause of pleasure and pain. They both consider "Ātman" as self, soul that is innermost essence of any individual being. Further, they both consider self-knowledge as the means of liberation, freedom and bliss. The difference between Samkhya and Advaita is that Samkhya holds there are as many "Atmans" as there are beings, each distinct reality unto itself, and self-knowledge a state of Ipseity. In contrast, the monism theme of Advaita holds that there is one soul, and that the self of all beings are connected and unified with Brahman. The essence and spirit of everything is related to each self, asserts Advaita Vedanta, and each "Atman" is related to the essence and spirit of everything; all is one; self is Brahman and Brahman is self.