Aristotle firmly believed that we can understand the world around us by studying it. For instance we can see different evidence of "dogs" around us. We understand that different dogs still have common characteristics. We can use our senses and our logic to understand just what a dog is... that truth is gathered from the world around us. So unlike Plato, Aristotle believed that we can trust our senses.
Just as we can understand the physical world by studying, we can also understand the emotional or philosophical world by studying it. So to understand virtue and justice, we need to study it. Through this method, Aristotle was able classify logic, metaphysics, poetics, ethics etc as separate classifications.
This ability to classify items led him to start the classification of animals, breaking them up in to different categories.
From his system of classification he developed a syllogism. This is where you have two truths which lead to a third truth. So you'd have premise, premise, plus conclusion. This ability to reason is what separates us from all other animals.
Not all that Aristotle believed and put together in his writing was accurate, but so much of what he did think and learn still holds true or relevant.