Aristotle has written a lot about the big questions in philosophy, and for that he used his ability to observe and think to an extend that was not seen before. He is not simply using the possibilities of the human mind, but also describes the laws of thinking in his works about logic. His work includes ethics, politics, scientific methodology, logic and more.
Aristotle criticizes Platos focus on the ideas and the isolation of them in a separate world. He does not agree with lifting the ideas above the world of sensory perception and points out the importance of the real objects for the ideas. Plato considers the ideas to belong to a real, but separate world, wich he can access with his mind. He gives only little attention to the worldly things, because he sees them as mere shadows of the idea that stands behind them. He wants to turn away from those shadows and direct his attention directly on the ideas. This is visualized in his allegory of the cave: We can only turn to the real world if we turn away from the wall with the shadows. It can be assumed that Plato was really capable of seeing the ideas in his mind, without having to look at a perceivable realization of them in the material world.
Aristotle however stresses the importance of sensory perception for the human being. An idea is not complete and not noticeable without a representation that is noticeable for the sensory organs in our body. Idea and object belong together, they form a unit, and for him the object is not just a shadow of the idea. He closely observes the worldly things accessible to the human senses and begins an extensive categorization. The ideas do exist for Aristotle: not in a separate world, but in the objects themselves and in the mind of a human who observes his surroundings. The objects of the world appear, and man gives them names. Humans analyze, categorize, define concepts, and assign everything to one concept or another. Humans process perception with their thinking. Doing so, they intuitively capture the idea that belongs to an object.
Compared to Plato, the worldview of Aristotle is much closer to that of modern society, because we also commonly consider our sensory organs as the primary gateway to the world. With our mind we are then adding an idea or concept to the perception in a second step. But without sensory impressions, our mind remains empty and does not find any ideas. The ideas are a part of reality, but they are neither innate to a human nor accessible in some separate world. They are united with the perceivable object, and can only be found when we think about what we observe. This way, Aristotle unites the two worlds and overcomes the dualism in Platos worldview.
According to Aristotle, all things that leave an impression on our sensory organs consist of two elements, that always occur together: The matter and the form. Things require matter, in other words a material, and a form, in other words an arrangement. The water in the glass is matter, and it takes the shape of the glass. To the form we also count that water changes its state from solid to liquid to gaseous at specific conditions, and that it can form solutions with other matter. The form includes both the formation and the attributes, the potential that something has. That becomes especially significant for organic things. The form of a chicken egg contains the possibility that a chick hatches. The form of a chestnut does not contain that possibility, even though it is made of the same matter (for example protons , neutrons and electrons). Instead, it can become a chestnut tree.
In his categorization Aristotle begins with two main groups. The first includes all things without a soul, everything that is not alive. Things in that group do not have the option to change, they do only get changed when something impacts them from the outside. The second group includes all living things. They have the option to change and move on their own initiative embedded in their form. Within that group, Aristotle differentiates animals and humans. We can also count plants as another group here. Alternatively, we can differentiate the realm of minerals, the realm of plants, and the realm of animals. Mankind is then a subgroup within the animals. This is open for debate. What we can agree on is that we will be quite upset when we cannot assign an object to one of those main categories. We commonly assign things to be either alive or not. However, in todays world it is not so easy find words for what actually defines the main groups.
We define categories based on what something is capable of or what it does. To belong to a category, certain abilities are necessary. Plants can consume food, can grow and reproduce. Animals can move and sense feelings on top of that. Humans do also have the ability to think rationally. Aristotle writes about a vegetativ soul, a sensitive soul and a rational soul of reason, and a human has all of them. Additionally, Aristotle was also interested in the relationship of concepts and investigated and described logic. Logic gives the rules how concepts can be connected with each other. We find those rules through an observation of our own thinking. They reflect the laws of the perceivable world in our mind. As a result, logic and mathematics are suitable to describe and predict the outside world.