There's a terseness about Sankara when he is at his best. The decks are being cleared and the cannons run out as it were. In Tai Up. II.i.1 the well known analysis of the definition of Brahman as satyam, jnanam, anantam is unpacked. First the opponent open up with an attempt at dismasting. The notion of the limited individual achieving unity with the unlimited is evidently incongruous. Not so says Sankara. It is a question of realisation. One is stuck in a conceptual framework much like the person counting the number of people that forded a river. He counts in a successive manner and finds only nine have made it over. Where's the tenth man?, he asks. When he shifts to counting inclusively the complete number is clear to him. Our normal and usual way of counting objects in the world out there is successive but that is not always adequate. I can remember the discussion about whether the year 2000 really marked the millennium because Dennis the monk (Dennis the Short) started from the year 1 when devising his chronology. A year had not elapsed then, so back to year zero if you please which makes our space odyssey 2001. Interestingly like the concept zero which was not available to height challenged Dennis (remember that meme) the concept of non-incongruity between the Jivatma and Brahman has also to be inculcated through rigorous analysis.
The individual soul, though intrinsically none other than Brahman, still identifies itself with, and becomes attached to, the sheaths made of food etc., which are external, limited, and composed of the subtle elements; and as (in the story) a man, whose mind is engrossed in the counting of others, misses counting himself, though that personality is the nearest to him and supplies the missing number, just so, the individual soul, under a spell of ignorance, that is characterised by the non-perception of one's true nature as Brahman, accepts the non-Selves, such as the body composed of food, as the Self, and as a consequence, begins to think, "I am none other than those non-Selves composed of food etc." In this very way, Brahman, that is the Self, can become the non-Self through ignorance, there is a non-discovery (in the story) of the individual himself who makes up the requisite number, and just as there is the discovery of the selfsame person through knowledge when he is reminded of that personage by someone, similarly in the case of one, to whom Brahman remains unattained owing to his ignorance, there may be a discovery of that very Brahman by realising that omnipresent Brahman to be non other than one's own Self - a realisation that comes through enlightenment consequent on the introduction of the scriptures.
koshas is a short account of the coshas/cosas/koshas.
There are 5 of them and they are part of the samkhya (meaning counting) ancient cosmological system. There are 20 elements or is it 25, anyway the analogy of counting as in the Tenth Man is maintained. The Koshas are the different ways of viewing the human constitution or schemas in terms of the spiritual, vital airs (pranas of yoga), biological, mental, bliss body. By focussing on the elements of the human constitution we may forget to consider the 'binding problem' as it is called in western epistemology. What pulls the whole together?