The following is taken from the wonderful text Sadhanai Saram written by Sri Sadhu Om. My comments are in italiscised red:

Duality only appears due to a lack of self-enquiry:

156. Know that the world, soul and God have all seemingly come into existence only because of our pramada or slackness in Self-attention.

It is the unreal and illusory ego that sees the (illusory and unreal) world:

157. The entire universe composed of the five elements, earth, water, fire, air and space, appears only due to our error of mistaking as ‘I’ the ego that rises from Self like a spark rising from fire, and which appears as different from Self whose nature is Being, and as identical with the mind, whose nature is rising.

Self-enquiry ends the ego illusion and duality:

158. The entire appearance of this world that is seen, including we (the individual or jiva) who see it, is a mere false appearance like a dream. If we keenly scrutinize the source (the real ‘I am’) from which the seeing ego rises, and thereby enter the heart and firmly abide there, the reality will shine forth (and the appearance of the triad – the seer, seeing and object seen – will disappear).

There is no desire or fear in the Self (or in Self-realisation), both of these being due to ignorance/ego or illusion:

159. If we attain the otherness-free knowledge (ananya jnana), that this world is nothing but our own Self, seen wrongly in our self by our self through our power of imagination, then the delusion of desire for or fear of the world (the objects we see in front of us), will never rise again; and we will merge in and become one with Self. This alone is our natural state.

The formless Self alone exists:

160. Self alone exists. Except oneself, nothing exists. But if one takes this pure consciousness, which exists and shines as ‘I’ to be the body, every thing will assume a form and appear to exist. If one inquires, “Is this ‘I’ only the body, or is it something else?” and thereby sees the true nature of “I,” every thing will cease to exist. See thus.

A summary and reiteration of what has been stated above:

161. If you ask, “For what reason is it said, that if one inquires and knows oneself, this entire world would disappear?” The reason is that the result at tained by the inquiry “Who am I?” is the destruction of the unreal individual (jiva), the ego, who is immersed in activity due to his taking the body to be himself. (That is, since the entire world-appearance depends for its seeming existence upon the ego who sees it, and since the ego itself has no real existence of its own, when the real ‘I’ is known, the ego will be found to be truly non-existent; where upon, the world-appearance will vanish, having no one to see it and thereby to give it a seeming exis tence).

The false ‘I’ is the ego, also known as maya. Enquire into the ‘I’, that is, do self-enquiry, and this illusion will vanish, along with the body, the mind and the world. Compare to Revelation Verse 31.

162. The ‘I’ which cannot shine without being joined with a body-form, is alone the ghost-like ego ‘I’, the great illusion (maya). If one courageously and without fear inquires, “Who is this I?” it will become non-existent, and along with it everything else will cease to exist.

163. If this ‘I’ the self-rising ego-appearance, is destroyed by the inquiry “Who am I?”, then everything else, which till then appeared only in him (in the mind) but which was seen as if existing outside of him, will cease to exist. The ever-existing Whole or purna, which then shines forth as ‘I-I’ the blissful existence which neither appears nor disappears, is the real Self, the true import of the word ‘I’.

The true Self does not come and go, unlike the body and mind and world:

164. The ‘I’ which is the Whole, which is the true import of ‘I’, which shines by its own light without appearing due to the functioning of the five sense-knowledges (and disappearing when they cease to function); which is the exalted and ever unleaving experience of Self, and which is the true nature of everyone, alone is the one non-dual reality; it alone is true knowledge.

The true nature of Jnana (knowledge) is simply the Self. The Self is itself the nature of Knowledge.

165. This true knowledge is not anything other than oneself; it is our own existence-consciousness ‘I am’. Why should we suffer by imagining and seeing the soul [Jiva or ego], world and God in this knowledge? Abiding firmly as mere Being, experience this knowledge correctly; then what thing other than oneself will appear there?

True destruction of maya/duality occurs with destruction of the ego:

166. The destruction of the entire universe together with the space and other elements that constitute it is not the state of absolute destruction (pralaya). The state in which the wicked ego, which rises in the form of the feeling “This body of flesh is I” (abhimana), drowns in Self and is destroyed entirely, is alone the glorious state of absolute destruction (maha-pralaya).

167. After being destroyed in the ordinary destruction at the end of an eon (yuga-pralaya), the heaven and earth will appear again at the proper time. The state in which the Fire of Knowledge (Jnanagni), which blazes forth on being ignited as the churning process of investigation “Who am I?” pervades and shines everywhere, having destroyed the feeling, “I am this body” (dehatma-buddhi), is alone the absolute destruction (maha-pralaya); be cause the ego, which is thus destroyed by the Fire of Knowledge (Jnana) will never appear again.

168. The “I,” which continues to live even after the mind has died (as a result of the above said inquiry “Who am I?”) is indeed the Supreme Reality (Brahman), which alone exists after the universal destruction (pralaya). The mind expanding as everything, having risen from the heart (that is, the mind slipping down from the true state of Brahman due to inadvertence or lack of Self-attention (pramada), and thereby becoming the cause for the creation and sustenance of the false appearance of this en tire universe), is not the state of true power; the mind merging and abiding firmly in Self is alone the true state of supreme power.

Note by Sri Sadhu Om: The supreme and truly divine power is not the power of the mind (maya), which creates and sustains the false appearance of the universe, but is only the power of Self-abidance, which reveals the truth that illusion (maya) is ever non-existent.

The cause of duality and of liberation:

169. Your appearance as an ego, an individual soul who rises in the form “I am this body,” alone is the root-cause for the appearance of this unreal world. If this root-appearance, the rising of yourself as an individual soul, does not rise even in the least, that state will be the state of liberation, which is completely devoid of the imaginary appearances of the world and God.

170. If one has the ability to deny oneself and thereby to destroy the rising of the ego in its very source, what other tapas need one perform? The real Self, the source in which the ego thus subsides and dies, alone is the state which is worth to purchase and attain by selling (or renouncing) all the three worlds.