The school of thought or sidhaanta expounded by Sri Adi Sankara, is known as Advaita. Greater thinkers who lived before the time of Acharya have also dealt with it. Wise men who came after Shri Acharya have also written profusely about Advaita pouring into their writings their own experience (Swaanubhava), of the Advaitic truth. There are such works not only in English, but also in Tamil, Kannada, Telugu, Marathi and Hindi. Some of them are original works of Advaita. Persons belonging to other schools of Hindu religious thought and persons professing other religions have also written on Advaita, out of the abundance of their rich spiritual experience. Some of the names that come to mind are Tattvaraya Swami, A Madhava, Mastan Saheb, a Muslim and Vedanayagam Pillai, a Christian. In recent times we have the example of the late A.V. Gopalacharya, who has written a number of treatises and essays on Advaita.
It is worthy of note that whatever their mutual differences may be, all thinkers belonging to schools other than Advaita, are one in their attack on Sri Adi Sankara's views. This should be regarded as a tribute paid by them to Sri Sankara Bhagavatpada. Each of them singled out Advaita, as expounded by Sri Acharya, as the only system worthy of taking notice of for the purpose of criticising. According to Advaita, the ultimate bliss is the experience of non-difference between the Jivatma and the Paramatma. Acharyas of other schools of thought would wish to have at least a tract of distinction between the two so that the Jivatma, standing apart, may be able to enjoy the realisation of the Paramatma. Thus the difference between the several systems of Hindu religious thought is slight, as all are agreed upon the ultimate realisation of the Supreme. But when it comes to a question of expounding each system, this difference got magnified to the point of violent opposition. And yet we find that in their ultimate reaches, all of them speak the language of Advaita. This shows that the expansive heart of Sri Adi Sankara accommodated all views of the ultimate reality and all approaches to it. Though other systems quarrel with Advaita, Advaita has no quarrel with any.
The catholicity of Advaita is also evident from the fact that pronounced Advaitins like Vachaspati Misra, who lived about one thousand years ago, Vidyaranya and Appayya Dikshitar wrote encyclopedic works on other systems with the fidelity of exposition rarely equaled and much less excelled by the protagonists of those systems themselves. Appayya Dikshitar says that as God's grace is required to reach the Ultimate Reality, and as that grace can be obtained only through Bhakti, he was expounding the other systems which promoted this Isvara-bhakti.
According to Shri Adi Sankara, no school of thought is foreign to Advaita. In the scheme of the path to realise Adviataanubhava, every system contributes an essential step and so Shri Sankara used the truths of each of them and pressed them into his service. By its very name, Advaita negates duality and dissension and comprehends every warring sect and system into its all-embracing unit. In fact, the survival of Hinduism is itself due to this Advaitic temper, which sees no distinction between Saivam, Vaishnavam and other denominations. Shri Adi Sankara underlined the essential unity of all Sampradaayas and sects and saved Hinduism from disruption. All denominations have the common Vedic basis. By bringing to our minds all the great Acharyas, we can acquire that peaceful frame of mind and develop that catholic temper and universal accommodation characteristic of Shri Adi Sankara and of the Advaita Vedanta he expounded, which will enable us to live in peace and amity, so essential for securing universal welfare.