Louis De Raeymaeker in his book The Philosophy of Being (pub. 1946) refers to Shankaracarya in a footnote on page 147 in the chapter on the static order of being.
The Indian school of Avaita (Shankaracarya, circ. 788 - 820) which allies itself to Brahmanism, maintains likewise that the world is real only because it is identified with the absolute, Brahma. Limitation and multiplicity derive from a potency, Maya, the great sorceress, who dwells in the heart of reality. They are only an illusion, an effect of ignorance, of imperfect knowledge, Avidya. Whoever places himself at the true point of view of the absolute, Brahma, enters into possession of the full wisdom, Vidya, which makes Maya and Avidya disappear and this man enjoys the beatific vision of all things in the perfect simplicity of Brahma.
The Brahmanism referred to here is related to the ancient Vedic religion and its practices such as the Fire Ceremony (Agnihotra) and its philosophic foundations in ancient Upanishads such as the Brhadaranayka and Chandogya, both of which have commentaries written by Shankaracarya. Avaita (commonly written Advaita) in its modern form stresses the philosophical rather than the ritual. Vidya and Avidya are simply knowledge and ignorance. The world is unreal only when it is taken to be a free-standing, self sufficient entity. It is real as manifestation. This is the usual formula given to allow a bearing to be established on the problem of creation, it is not a closely argued final position. The Real in Advaita is that which is changeless and necessary, the unreal is changeful and contingent. The well known argument from illusion as operative in Western philosophy is not an issue in this connection.
De Raeymaeker is accurate on this point though he succumbs to the fanciful allegoricisation that poetic Hindu texts go in for. In any case it is just a remark in passing, his chief focus is on Spinoza who offers an analogous account of ultimate absorption into unity of being.
The modes (of finite being) are founded on, and every distinction is effaced in, infinite Nature; the opposition between possibility and reality , between essence and existence, disappears in the fundamental unity of pure Being.
Interestingly De Raeymaeker in his discussion of The Possible, The Problem of the Foundation of Intrinsic Possibility and The Problem of the Relation of Extrinsic and Intrinsic Possibility covers in his closely argued methodical way the grounds for the more compressed insights of Shankaracarya on the issue of the Buddhist Idealist claim that a thing’s existence is based on whether it can be known or not. If it cannot be known as per the standard idealist view we are only acquainted with the contents of a mental experience, then the existence of a thing as possible or impossible cannot be determined. Shankaracarya’s counter is that it is the application of valid means of knowledge (pramanas) that determines whether a thing is impossible or not.
What is known through any one of the means of knowledge, such as direct perception etc., is possible, and what cannot be known through any one of these means of knowledge is impossible. In the case under discussion, the external things are known individually by the respective means of knowledge;...(B.S.B. II.ii.28)
Denying the possibility of your empirical acquaintance with something says nothing about whether it is possible or not and in fact makes it impossible to say whether or not it is an existent.
De Raeymaeker (pg.91):
It is not the abstract, as such, which can exist, but the concrete individual; it is not the abstract essence “man” which is possible, it is this or that man, Socrates or Plato.