What is the significance of different dimensions of reality to Plato, Aristotle, Plotinus and Sir Philip Sidney ?
The context of your question, in relation to Sidney's Defence of Poesie, leads me to suppose the interest in these four great thinkers relates to the place of poetry and poets in their perceptions of the dimensions of reality. In fact, this is an intriguing perspective on their thoughts and one that is not popular enough to be widely presented and is therefore not readily researchable. In the limited format offered by eNotes, we can skim the surface of the question by starting with Sidney. We'll work briefly backward from Sidney to Plato, though Plato came first, followed by Aristotle (Plato's student) and Plotinus, and then Sidney.
One briefly stated aspect of Sidney's understanding of--and poetic aesthetic philosophy of--the different dimensions of reality was based on his belief in the divine and human realms of reality.
Sidney conceived of the poet as the recipient of divine knowledge whose task it was to formulate divine realities into Nouns, or individual Characters, ultimately presenting the true picture of a given divine reality and to formulate these Nouns into corresponding Verbs that conveyed the virtuous Actions of the Character. All possible variation on a Noun, ranging from true and virtuous down to false and vile, were presented to illuminate the true divine idea. According to this, he conceived of poetry that gave personality to, for example. Love, and that presented actions that accorded with the personality.
See whether wisdom and temperance ... all virtues, vices, and passions so in their own natural seats [are] laid to the view, that we seem not to hear of them, but clearly to see through them ... these other pleasant fault-finders, who will correct the verb before they understand the noun, and confute others’ knowledge before they confirm their own; I would have them only remember, that scoffing cometh not of wisdom; ... (Sidney, Defence of Poesie)
This one aspect of Sidney's philosophy and aesthetic of poetry illustrates his understanding of the different dimensions of reality. Sidney noted two dimensions of reality, the divine and the human. Sidney saw the poet as the intersection between the dimension of the divine and the dimension of the human, and he saw poetry as the road map to the truth and virtue residing in the divine.
The scope and influence of this concept is best illustrated by another poet who was a contemporary of Sidney's, whose works are more well known than Sidney's and whose comedies were formulated around this concept. The comedies of Shakespeare proffer a full range of variations of a divine truth, say, Love, through a full range of characters who display a full range of actions representing love, while the hero (who is ironically usually the heroine) demonstrates the true virtue of love and the true actions attributed to love. As You Like It, with Rosalind and Orlando, is a prime example of how Shakespeare embodies Sidney's principle.
[Poetry] is that feigning notable images of virtues, vices, or what else, with that delightful teaching, ... not speaking table-talk fashion, or like men in a dream, words as they changeably fall from the mouth, but piecing each syllable of each word by just proportion, according to the dignity of the subject. (Sidney, Defence of Poesie)
Because Plato wrote in a dialogic form (in dialogues between two or more persons) that argued and debated the different dimensions of reality and poetry--among many other important ideas--Plotinus (204-270 C.E.) knew that Plato needed to be interpreted so the ideas might be crystallized in an accessible form. This interpreting was no small task since there is considerable debate as to whether Plato changed his mind, disagreed with some of the arguments presented in the Dialogs or was playing devil's advocate against his own ideas; thus Plato's approach to arguing ideas in the Dialogs allowed his concepts to be presented in their full scope, with arguments against the concepts already addressed.
[Plato's] readers are not presented with an elaborate system of doctrines held to be so fully worked out that they are in no need of further exploration or development; ... (Stanford Encyclopedia of Philosophy)
When considering Form, Plotinus agreed with Plato--as he considered himself to be a Platonist--in understanding there to be two different dimensions of reality: (1) the ideal dimension where Form existed ideal characteristics of truths (what we now called "abstract ideas") and (2) the human dimension where the ideal was deprived and degraded. Plotinus postulated the existence of the "One" from which all truths emanated. One emanating truth was Beauty as expressed in art, and art includes poetry. Consequently, the One inspired the poet who expressed ideas emanating from One. These ideas expressed in poetry are separated only slightly from the human dimension but are separated to the greatest degree possible from the dimension of non-divine, non-human matter.
Aristotle approached the question of the dimensions of reality from a perspective focusing on the natural and on nature. A good deal of Aristotle's philosophy, as any freshman-year Great Books student will confirm, stemmed from studies of animals, especially in their natural habitats. Still, Aristotle gave equal dominance to both poetics and rhetoric (Poetics and Rhetoric).
For Aristotle, ideals and phenomena/matter are separate dimensions of reality. Yet, his concept differs from Plato's (Plato was Aristotle's teacher) in that ideal perfection, which he calls "essence" and which is the product of the unmoved mover, is that which gives phenomena/matter its realization (i.e., reality or expression) of shape, form and purpose. In other words, while Aristotle also describes two dimensions of reality, the one (essence) is integral to the other (phenomena) since it (phenomena) cannot be anything other than vague "stuff" without the realizing potency of the first (essence): essence empowers and provides realization (reality) to shapeless, formless, purposeless stuff of phenomena/matter.
Aristotle viewed poetry--and all art--as that which imitates human characteristics and actions, both the noble and the ignoble. Thus he recognized two classes of poets: individuals who were noble (possessing virtue and elevated thinking/contemplation) and wrote of characters, or "agent," who were greater than they themselves were, and individuals who were ignoble (lacking virtue and elevated thinking/contemplation) and wrote of vile "agents" who were worse and lesser than they themselves were. Aristotle postulated that humans are the closest to the original and originating unmoved mover--who brought all into being yet was never brought into being and never changes--therefore imitation of virtue in idea, word and action by noble poets of noble agents is imitating the characteristics and qualities of the unmoved mover; this is a concept Plotonius developed later as the "One."
For Aristotle, this imitation in the words of poetry is valuable for imparting learning about human virtues, which, again, are the imitation of the characteristics of the unmoved mover. Aristotle held that imitation is a natural impulse of humans and that it is enjoyable to humans. Thus he posited viewing imitation as a prime way of learning because nobleness, in its various aspects of breadth and depth, was being demonstrated for the viewer. Thus Aristotle perceived noble poets as being inspired.
Aristotle's position on poetry (and all arts) implies that poetry unites the two dimensions of reality that he perceives and it does so through imitation (called mimesis, meaning "imitation") of the noble qualities of the divine as these are the defining traits of the unmoved mover who is the embodiment of virtue in contemplation.
Plato was decidedly against poets and poetry and postulated a dichotomy between two dimensions of reality, that of Ideas/Form and phenomena/matter. For Plato, ideas and phenomena exist independently. The easiest illustration of this concept is to describe the independence of the idea of Soul from the phenomena of Body: Soul and Body are so very much different dimensions of reality that Soul exists without Body and Body (such as the body of an aardvark) can exist without Soul.
This perception led Plato to view the imitative arts, such as poetry, as inferior and misleading. He postulated that poetry represented a vague, twice-veiled understanding of the perfections of Ideas that led humans astray, distracting them from the truth about Ideas and preventing them from contemplating the realities of the higher dimension.
While a dichotomous universe divided between Ideal and Matter persisted from Plato through Sidney, it was Sidney who elevated poetry to the level of divine communication through divine inspiration regarding divine truth, which humans wanted, as well as needed, to hear and to learn from. It was Sidney who, perceiving the dimensions of reality to be bridged by poets, lauded poetry as the vehicle for showing all aspects of an Idea (for example, Love) in all its noble and ignoble representations so that humans would know what to resist and avoid, what to yield to and accept, and what to imitate as the True aspect of that Idea.
This is a very large topic but I will try to give you a brief overview of each philosopher's thought on the dimensions of reality:
Plato did not think reality came through any of the five senses because the senses are untrustworthy. Reality exists on a higher plane, available only through one's intellect: "The philosopher is in love with truth, that is, not with the changing world of sensation, which is the object of opinion, but with the unchanging reality which is the object of knowledge."
Aristotle believed that motion and flux revealed reality: "Since nature is a principle of motion and change, and since our inquiry is about nature, we must not overlook the question of what motion is. For without understanding motion, we could not understand nature."
Plotinus postulated that "it is by virtue of contemplation that all existents are said to be united as a single, all-pervasive reality." He explains, "Pleasure and distress, fear and courage, desire and aversion, where have these affections and experiences their seat? Clearly, either in the Soul alone, or in the Soul as employing the body, or in some third entity deriving from both. And for this third entity, again, there are two possible modes: it might be either a blend or a distinct form due to the blending."
Sir Philip Sidney believed that poetry created a separate of reality, privileging perception and imagination as a means of understanding: "Each excellent thing, once learned, serves for a measure of all other knowledge”