Question: I have seen the following argument in several places:

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> Consciousness cannot be an illusion, and it surely exists.
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> An illusion is perceiving something to be true when this something is not actually the case. "Perceiving" and "being conscious of" can be taken to be synonymous, so saying that consciousness is an illusion is a contradiction.
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The argument is very strong as it relies on semantics.

**What is the first written philosophical account of this argument?**

Answer: 
1) Taken your question literally: One does not need to argue that consciousness exists. Everybody when awake is conscious and experiences consciousness. 

One can discuss whether every perception is a conscious perception. Because we know that humans can also be triggered by unconscious stimuli. This kind of stimulation is used by marketing mechanisms or by priming, applied in an experimental context of cognitive psychology.

2) Hence the question is not, whether consciousness exists. The questions are:

* How to define conscious mental processes and how to establish the border to unconscious processes?
* Why are some mental processes conscious while others are not? What is the function of conscious processes?

As an introduction I recommend *Koch, Christof: Consciousness - Confessions of a Romantic Reductionist.* (2012). Chapter III reviews some definitions of conscious processes, taken from different view points. 

Whether a person is conscious or not and to which degree he is conscious can be derived from its behaviour. That’s a method used by paramedics using a checklist. 

A more refined criterion to decide about the conscious state of a person is to check the activity of the neocortex and the thalamus. 

A more philosophical definition of being conscious is to register how things seem to us (qualia). 

In any case, conscious mental processes are restricted to the neocortex.

3) An early classification of states of consciousness is given by the Mandukya Upanishad from the beginning of the common era. The Mandukya Upanishad distinguishes four quarters of atman (= the self):

* Vaisvanara: “situated in the waking state, perceiving what is outside […]” (verse 3)
* Taijasa: “situated in the state of dream, perceiving what is inside […]” (verse 4)
* Prajna: “situated in the state of sleep – deep sleep is when a sleeping man entains no desires or sees no dreams […]” (verse 5)
* “They consider the fourth quarter as perceiving neither what is inside nor what is outside, not even both together; not as a mass of perperceptions […]” (verse 7)
Upanishads would be the earliest. One of the *Mahavakyas* ('Great Sayings') of the Upanishads (there are 4 in total) is *Prajnanam Brahma* - Consciousness is Brahman. It appears in the *Aitareya Upanishad* III. i. 3. The *Aitareya Upanishad* is part of the *Rig Veda*, considered by many as the oldest Veda. Most consider the Vedas at least 1500 BC. Verses 2-3 says (Swami Nikhilananda translator):

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> It is the heart and mind. It is [known in accordance with its different functions, as] consciousness, lordship, knowledge [of arts], wisdom, retentive power of mind, sense knowledge, steadfastness, thought, thoughtfulness, sorrow, memory, concepts, purpose, life, desire, longing [for sense-objects]: all these are but various names of Consciousness (Prajnanam).
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> He is Brahman, He is Indra, He is Prajapati: He is all these gods; He is the five great elements--earth, air, akasa [space], water, light; He is all these small creatures and the others which are mixed [with them]; He is the origin [of the moving and unmoving]--those born of an egg, of a womb, of sweat, and of a sprout; He is horses, cows, human beings, elephants--whatever breathes here, whether moving on legs or flying in the air or unmoving. All this is guided by Consciousness (Prajnanam), is supported by Consciousness. The basis [of the universe] is Consciousness. Consciousness is Brahman. 
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