Introduction to Idealism

The first known idealist is Plato. He  introduced the ideas and asserted that human’s mind can understand the ideas, the question is what is ideas? I am giving you a example, when I am saying that I am looking at black horse I am using two concepts black and horse and with putting together these two concepts I am pointing an existing case, a black horse but what pure horse is besides its several attributes for example blackness, tallness, shortness or whatever attributes. This horse without color without length without breed and all other characteristics doesn’t exist. Can you imagine? Yes it doesn’t exist. So what is horse-ness? Where does it come from? This is the main motivation of Idea theory of Plato. He says there is forms/ideas world that all ideas live there and all real horses for example our black horse is a imperfect copy of the idea of horse.