Plato was a philosopher in Classical Age Greece, and was a close friend and follower of Socrates. Plato, unlike other philosophers of his time, did not think about the natural world, but rather moral qualities and how men should act. This is how he taught, and I’ll give an example through blades.
If you remember the last post, I listed the three main types of blades, the Rapier, Katana, and Kopis. These are all incredibly different. Yet they are all swords. If you showed an animal a Rapier and a Kopis, would the animal recognize them both as swords? No. Swords come in all shapes and sizes, from Rapiers to Ulfberhts to Katanas to Kopii. They all must have one thing in common that makes them a sword. Let’s call this thing they share in common “swordness” (for lack of a better word). Now a Kopis has extra weight behind the edge for harder blows, but extra weight is not part of the swordness factor: this is proved by the Rapier, which has practically no weight behind the edge. What about a pointed end? Is that part of swordness?
Most swords are rounded up into a point, but there are many swords that have a flat end, for example a type of machete. Does a sword have to be curved? No. Does it have to be made of steel? Nope. Does it have to be very long? Uh-uh. Every sword has a feature that is not definitive in it’s swordness; every sword has something that another might not have. This means it is impossible to to imagine “swordness” as an actual object. No matter what you imagine as this object, another sword, it will still have something that it does not need to have in order to be a sword. I will leave it up to you to find the one thing that is common to all swords, and all swords have to have in order to be a sword.
This goes as well for qualities, such as Justice, Kindness, etc.; you can give examples of Kindness; that man just gave the poor beggar fifty bucks: but you do not have to be giving money, you do not have to do it to a beggar, it does not have to be fifty bucks, in order to be kind to someone. But there is something in common with all these examples of kindness; there is a definition of “kindnessity”, just as there logically must be something in common to all swords, there must be something in common to all acts of kindness, of all acts of Justice, of all things that are beautiful. Plato said that men should use their reason not to find the natural things of this world, what this is made of and why it works, but we should use our reason to find these things that are pure Justice, Kindness, Beauty, etc., and then use these and live by them.