Platonism, as propounded by the ancient Greek philosopher Plato, asserts that the realm of Forms or Ideas constitutes the true reality, while the physical world is a mere reflection or imitation of these Forms. Thus, existence is the domain of the Forms, while non-existence is the absence of these Forms.

Parmenides argued that the fundamental principle of reality is Being, which is eternal, unchanging, and indivisible. He famously proclaimed, 'What is, is; what is not, is not.' This principle emphasizes the immutability and permanence of existence, contrasting it with the flux and impermanence of the physical world.

Nihilism is the philosophical doctrine that existence is ultimately meaningless and purposeless. It rejects the notion of objective values, truths, or moral principles, asserting that life is inherently devoid of inherent significance.

Non-Being, also known as nothingness or absence, is the metaphysical concept that represents the state of not existing. It is the opposite of existence and encompasses everything that does not exist, including things that never existed, things that no longer exist, and things that are impossible to exist.

Existentialism is a philosophical movement that emphasizes the importance of individual existence, freedom, and choice. It posits that existence precedes essence, meaning that individuals create their own meaning and purpose through their actions and choices, rather than being defined by predetermined essences.

Heraclitus famously stated, 'Everything flows,' encapsulating his belief that change and motion are the fundamental principles of reality. He argued that the world is in a constant state of flux, with everything undergoing perpetual transformation.

Absurdism is a philosophical doctrine that emphasizes the inherent absurdity and meaninglessness of existence. It posits that there is a fundamental disconnect between the human desire for meaning and the irrationality of the universe, leading to a sense of absurdity and existential angst.

Absolute Nothing, also known as pure nothingness or the void, is the metaphysical concept that represents the state of being that is completely devoid of any properties, qualities, or characteristics. It is the ultimate absence of everything, including space, time, matter, and consciousness.

Rationalism is a philosophical movement that emphasizes the primacy of reason and logical thinking in acquiring knowledge. It posits that knowledge is innate or can be derived through deductive reasoning, rather than relying solely on sensory experience.

Zeno's Paradox of Achilles and the Tortoise is a famous thought experiment that challenges the concept of motion. It posits a race between Achilles, the fastest man in Greece, and a tortoise, with the tortoise given a head start. Zeno argues that Achilles can never overtake the tortoise, as he must first reach the point where the tortoise started, then the point where the tortoise was after that, and so on, leading to an infinite series of smaller and smaller distances.

Absolutism is a philosophical doctrine that posits the existence of absolute truths, values, and principles that transcend individual perspectives and cultural relativism. It asserts that there is an objective moral order that governs the universe, providing inherent meaning and purpose to existence.

Plenitude of Being, also known as the totality of being or absolute being, is the metaphysical concept that represents the state of being that encompasses all possible properties, qualities, and perfections. It is the ultimate fullness of existence, where every conceivable aspect of being is realized.

Empiricism is a philosophical movement that emphasizes the primacy of sensory experience and observation in acquiring knowledge. It posits that all knowledge is derived from experience, either directly through perception or indirectly through reasoning based on sensory data.

Anaximander posited that the fundamental principle of reality is the Boundless or the Infinite (Apeiron in Greek). He conceived of the Boundless as an indeterminate, limitless substance from which all things arise and to which they ultimately return. The Boundless is both the source and the end of all existence.

Pessimism is a philosophical doctrine that posits the inherent evil and meaninglessness of existence. It asserts that suffering and pain are pervasive in the world, outweighing any potential joys or pleasures. Pessimism often leads to a negative outlook on life and a questioning of the value of existence.