Quote from Überschall

"Asking for highly philosophical manifestos in a stickfigure animation forums mostly populated by people below 15 years of age" by Überschall is a pretty good one.

Quote from Scarecrow

aldous huxley: the perennial philosophy

Quote from Automaton

Could anyone recommend some books that give varied opinions on philosophical subjects please? i.e. a book from one philosopher who believed in one school of thought and another from a philosopher who believed in a different one.

These are the subjects I'm interested in:
Empiricism / Rationalism (already have Hume's enquiry so really only need one dedicated to the rationalist epistemology)

Strong Atheism / Deism (already have the god delusion)

Metaphysics

Ethics

Ontology

Political ideologies (particularly important to me as a topic, but we're currently doing this in politics anyway)

Quote from Automaton

Feel like kind of a dick for asking this question about a philosophical book which is probably way too complex to simplify, BUT:
what's it about, overall?

Perennialism is a perspective within the philosophy of religion which views each of the world’s religious traditions as sharing a single, universal truth on which foundation all religious knowledge and doctrine has grown. Each world religion, including but not limited to, Christianity, Islam, Judaism, Hinduism, Taoism, Confucianism, Shinto, Sikhism and Buddhism, is an interpretation of this universal truth adapted to cater for the psychological, intellectual and social needs of a given culture of a given period of history. The universal truth which lies at heart of each religion has been rediscovered in each epoch by saints, sages, prophets and philosophers. These include not only the 'founders' of the world's great religions but also gifted and inspired mystics, theologians and preachers who have revived already existing religions when they had fallen into empty platitudes and hollow ceremonialism.

Although the sacred scriptures of the world religions are undeniably diverse and often superficially oppose each other, there is discernible running through each a common doctrine regarding the ultimate purpose of human life. This doctrine is mystical in as far as it views the summum bonum of human life as an experiential union with the supreme being that can only be achieved by undertaking a programme of physical and mental purification.

Aldous Huxley, who wrote a widely read book on the subject, defined the perennial philosophy as:

the metaphysic that recognizes a divine Reality substantial to the world of things and lives and minds; the psychology that finds in the soul something similar to, or even identical to, divine Reality; the ethic that places man's final end in the knowledge of the immanent and transcendent Ground of all being; the thing is immemorial and universal. Rudiments of the perennial philosophy may be found among the traditional lore of primitive peoples in every region of the world, and in its fully developed forms it has a place in every one of the higher religions
(The Perennial Philosophy, p. vii).

He also pointed out the method of the Buddha:

The Buddha declined to make any statement in regard to the ultimate divine Reality. All he would talk about was Nirvana, which is the name of the experience that comes to the totally selfless and one-pointed. […] Maintaining, in this matter, the attitude of a strict operationalist, the Buddha would speak only of the spiritual experience, not of the metaphysical entity presumed by the theologians of other religions, as also of later Buddhism, to be the object and (since in contemplation the knower, the known and the knowledge are all one) at the same time the subject and substance of that experience.
The Perennial Philosophy

and that in the Upanishads:

The Perennial Philosophy is expressed most succinctly in the Sanskrit formula, tat tvam asi ('That thou art'); the Atman, or immanent eternal Self, is one with Brahman, the Absolute Principle of all existence; and the last end of every human being, is to discover the fact for himself, to find out who he really is.
Aldous Huxley

According to Karl Jaspers:

"Despite the wide variety of philosophical thought, despite all the contradictions and mutually exclusive claims to truth, there is in all philosophy a One, which no man possesses but about which all serious efforts have at all times gravitated: the one eternal philosophy, the philosophia perennis."

And according to Frithjof Schuon:

It has been said more than once that total Truth is inscribed in an eternal script in the very substance of our spirit; what the different Revelations do is to “crystallize” and “actualize”, in different degrees according to the case, a nucleus of certitudes which not only abides forever in the divine Omniscience, but also sleeps by refraction in the “naturally supernatural” kernel of the individual, as well as in that of each ethnic or historical collectivity or of the human species as a whole.