"... Aristotle reacted against the extreme realism which he took Plato to be endorsing: the thesis of universalia ante res (Latin: “universals before things”), according to which universals exist in their own right, prior to and independently of their instantiation by sensible particulars. He advocated instead a more moderate realism of universalia in rebus (“universals in things”): While there are universals, they can have no freestanding, independent existence. They exist only in the particulars that instantiate them."

— https://www.britannica.com/topic/realism-philosophy/Universals

I think they are two different forms of realism, two different interpretations of reality.

Manthi Chatzimichailidou

Contemporary realism/antirealism may concern the ontological or existential status of (i) abstract entities (e.g. numbers, sets, universals) and of (ii) postulated unobservables (e.g. certain fundamental particles in physics). For (i) the question may include the issue of whether mathematics is discovered or invented. For (ii) the issue becomes whether postulated unobservables really exist or are just a useful heuristic or fiction for purposes of calculation or prediction. In the social sciences a question might be whether a type of social entity must be considered as a separate entity over and above the people that are its members, or whether moral relativism is true.

https://plato.stanford.edu/entries/realism/

https://plato.stanford.edu/entries/platonism/

Karl Pfeifer thank you a lot indeed for that interesting answer and for the links!

Plato's and Aristotle's Realism

Both Plato and Aristotle are considered realists in the sense that they believe that the world exists independently of our minds. However, they differ in their understanding of what reality consists in.

Plato's Idealist Realism

Plato's realism is often referred to as idealist realism. He believed that reality is ultimately composed of Forms, or perfect, unchanging, and eternal essences. Forms are the universal concepts that all concrete objects instantiate (or participate in). For example, there is a Form of Apple, which is the essence of what it means to be an apple. All individual apples are imperfect copies of the Form of Apple.

Plato argued that the world of Forms is the only truly real world. The physical world is simply a shadow of the world of Forms. It is constantly changing and imperfect. We can only access the world of Forms through our reason, not through our senses.

References:

• Plato, Republic, 509d-511e.
• Plato, Theaetetus, 176a-181b.
• Plato, Parmenides, 128a-132e.

Aristotle's Moderate Realism

Aristotle's realism is often referred to as moderate realism. He believed that reality is composed of individual substances, which are concrete objects with their own individual essence and properties. Substances are not perfect or unchanging, but they are nonetheless real.

Aristotle agreed with Plato that Forms exist, but he believed that they are embodied in individual substances. The Form of Apple, for example, is instantiated in all individual apples. However, the Form of Apple is not something that exists independently of individual apples.

Aristotle also believed that we can access the Forms through our senses. We can perceive the universal form of a thing by comparing and contrasting individual instances of that thing. For example, we can learn about the Form of Apple by comparing and contrasting different apples.

References:

• Aristotle, Metaphysics, Z.1038a35-b36.
• Aristotle, Categories, 1b25-2a10.
• Aristotle, Posterior Analytics, B.19-20.

Conclusion

Plato's and Aristotle's realism are two of the most influential realist theories in the history of philosophy. They differ in their understanding of what reality consists in, but they both agree that the world exists independently of our minds.

If you are going to make inquiries about reality, I recommend that you first work with impartial artificial intelligence and bibliographies. Immanuel Kant also has very good works on these subjects, but I would like to recommend you especially P. D. Ouspensky's Tertium Organum book should definitely be read.