Ātma is taken to be body, mind, sense complex (gross and subtle body), because of non discriminative knowledge about ātma and anātma (what is the real me and what is not real me). It is called ignorance (avidyā or añjanam). Avidyā is not absent of knowledge about myself, but it is the knowledge apposed to what myself really is. Avidyā is also called māyā, is the causal body (kārana-śarīra) of our gross and subtle body. It is called causal body because it is the caused of others two body.
Avidyā is anādi, beginningless
It is said to be beginning-less, because any beginning implies its prior absence. Māyā was always there as the power of creation of Brahman (consciousness which is beginning-less), therefore any creation is endowed with this causal body. It is called śarīra, being upādhi, an adjunct, because of which Ātma is Jīva, an individual who has the sense of doership, does karma, gathers punya-pāpa, which cause to be born again and again. So, this life of samsara has no beginning, and it does not exist by itself. Because māyā shaktī is not a separate independent entity apart from Brahman consciousness.
Avidyā is anirvācya (can not be categorised as real nor unreal)
If avidyā is a separate entity which has a reality apart from consciousness, means we are presenting two parallel reality which has no connection with ātma at all. But that is not how it is. Any ignorance, whether it is ignorance of an object or ignorance of ātma, does not exist independently, but depend upon ātma the consciousness. Ātma is the āśraya, the basis, that gives reality to this very ignorance.
Avidyā is neither independently existent like ātma, nor it can be dismissed as non-existent. Every experience's of each individual and even the entire experiential world is because of avidyā alone, therefore its existence can not be dismissed. But it is negate-able when the knowledge about the the Self arises. Avidyā is anādi, beginning less, but it come to end. Reality is that which has no beginning and no end. Therefore avidyā can not be categorised as real nor unreal. This is called mithyā. Just like a pot can not be stated as existent if you can see it is nothing but just clay, nor it can be dismissed as non-existent when you see the functional purpose of it.
When the kārana-avidyā in its own form, until the product arises, has no distinction. It is free from any division. Pure ignorance is when its product is not there and errors is not there. This kind of experience is when we are in deep sleep state and nirvikalpa, no knower-known difference, no experience of duality.
When is pure kārana-avidyā called śarīra?
It is called śarīra only when it's product in the form of gross and subtle body, entire experiential world and it's experiences are arise. It is the cause for ātma to have a status of being jīva the individual self.
Q: Are we this causal body?
No. When me ātma is not identify with kārana-śarīra, I am pure consciousness.
Because of two fold power of māyā, āvarana (veiling) and vikshepa (projection) śhakti, we are identify with all these limitation.
Now me ātma is independent of these three śarīra, though all three are not independent of me ātma.