Plato's Theory of Forms and Hume's Problem of Reality

Plato's Theory of Forms

It is not possible to give a common definition of objects that change continuously, unlike other beings. These beings were called "ideas." Things are separate ideas, but they are related. Throughout Plato's work, the term "idea" is mentioned, first in the Euthyphro about the idea of piety, where it means a model or criterion of truth. However, its definitive formation is found in the dialogues of maturity, especially in the Republic. The theory posits that ideas exist apart from particular things. They are not a "commonality" within things, nor are they concepts or mental representations. These are entities that have real and independent existence; every idea is a substance (ousia). The theory implies a doubling of the world, with a separation between the visible world of particular things and the intelligible world of ideas. This is illustrated in the Republic through the allegory of the good, the simile of the line, and the myth of the cave. The gap between the worlds and their duplicity leads to a crisis in Plato's late works, as seen in the so-called third man argument: if things are what they are because they share ideas, there should be a third entity, other than the things and ideas, that makes the sharing relation possible. This, in turn, must have something in common with the above, leading to an infinite regress.

Hume's Problem of Reality

Hume investigates the nature of the evidence of any real existence and matter of fact. We have impressions and memory to ensure the reality of the present and our past. The problem lies in the future, because we cannot have any impression of it. However, some future events seem obvious. For example, if we see someone put their hand in a fire, we are sure they will burn. How is this evidence based? All reasoning concerning matter of fact seems to be based on the relation of cause and effect. Only through this relationship can we go beyond our memory and senses. The causes and effects cannot be discovered by reason but by experience. All arguments based on experience rely on the similarity found in natural objects, which leads us to expect effects similar to those seen following such objects. Practice leads us to believe that the same event will repeat. Our certainty about the future is, therefore, based on custom.