Philosophy was long regarded as the sole method of thought that would explain phenomena, be it imaginary, concrete, celestial, or terrestrial. Aristotle was a biologist, if we look at his work from today's perspective. He was mainly interested in the species that scattered all over the places. His concern was to collect as many different animals and plants as possible, so that he would have been able to talk about them with greater certainty. However, he was also a strong advocate of categorization and in order to categorize what he had collected, he needed to contemplate on what to put in this or that category or on what makes two things different. This contemplation made him a philosopher, as we understand it today.
His master, Plato, was no different in this respect. His basic concern was to understand the universe and humans within it. But his method was drastically different. For him, in order to figure out what things are and how they work, we needed to seek for universal ideas, which covers (or operates) everything. Hence Plato came up with the idea of Ideas, namely the universal realities. The world surrounding us could only be an aggravate of reflections of those Ideas because it lacked perfection. It was corrupted. To understand these reflections in their perfect state, we needed to contact with those Ideas.
For both of the founding fathers of the Western philosophy, Plato and Aristotle, philosophical inquiry inevitably involves in scientific investigation. Even though their approaches are different (some even claim that they are complete opposites), they shared the same concern: understanding the world surrounding us by way of investigation.