Philosophy is not limited to any particular subject matter. It covers art, science, law, morality, psychology, politics, language, and so on. The fundamental nature of the questions it asks The types of questions asked by philosophy are more fundamental than the types of questions asked within the other disciplines. For instance, philosophy of religion asks not how to worship God, but whether God exists. The skeptical nature of the question it asks Philosophy differs from other disciplines in that it assumes nothing.
Philosophy takes nothing as being beyond doubt.
The types of questions asked by philosophy are not susceptible to empirical investigation. These questions cannot be answered by any sort of empirical observation or experiment. Instead, philosophy employs the method of argumentation in order resolve such issues. In other words, it is impossible for a valid argument to have true premises and a false conclusion. If Doug is a genius, then he must be a philosopher.