The terms philosophus and philosophia were first employed by Pythagoras (in Cicero’s Tusculan Disputations) and roughly meant to Pythogoras as the pursuit of wisdom. The term philosophy then in turn changed to signify wisdom and the love of it, which was seen as the highest kind of knowledge. There are various kinds of knowledge, for example knowledge we gain via our everyday experience of reality for example historical, literary, knowledge about the physical world, knowledge about art etc. There are also other forms of knowledge that we gain by making use of our intellect and capability to reason in order to classify, analyze, and interpret etc. the knowledge we gain from everyday experience.
We try to explain knowledge gained from our senses in order to understand and gain insight about their causes and reasons. Knowledge from our capability to reason in turn yield systemized truths which we call sciences. For example, knowledge about the physical world gained from our senses are in turn intellectually analyzed to yield certain truths about the physical world and thus rightly called the empirical physical sciences (e.g. physics and biology).
Knowledge about certain geometrical structures gained via sense experience is reasoned and analyzed and in turn yield certain truths about them and rightly called the mathematical sciences (e.g. algebra geometry in this case etc.). There is also knowledge from our sense experience of our social enterprises as humans and from these we analyze, reason etc. and from it the social sciences emerge (e.g. economy, psychology, political etc.). These are called special sciences since they deal with whatever relevant causes of the particular topic they are concerned about.
There are also more general sciences whose domains are much more extensive than the special sciences. For example, we are also moral beings that are capable reasoning and from this moral and ethical science arise that need to incorporate understanding from physical sciences and social sciences.
In this sense, philosophy is seen as a general science and its main aim is to answer deeper and more extensive questions and in order to do so, rational enquiry and reasoning needs to be employed to understand the more ultimate reasons and causes of things. Here it is important to distinguish between philosophy and theology. Theology tries to answer and reason about ultimate questions by making use of reason that is aided and enlightened by Divine revelation (For example Christian theology is enlightened by Jesus, or Islamic theology by Mohammed etc.). Philosophy, properly understood, is thus different from theology and different from the special sciences and properly defined as: the science of all things through their ultimate reasons and causes as discovered by the unaided capabilities of the human intellect and human reason (See Coffey's, Ontology, Theory of Being).
Philosophy can be divided into practical (e.g. ethics, Logic, Philosophy of science, mind etc.) and speculative (e.g. metaphysics).
A few things about the philosophy of logic, metaphysics, the special sciences and especially the empirical physical sciences needs to be said to further distinguish between what kinds of sciences they are.
The empirical physical sciences studies things such as objects, substances, processes, organisms etc. and may be referred to as “real sciences” or “scientiae reales” as the Scholastics described it. Philosophy of logic studies thought as a process capable of attaining certain proofs and truths. The study of logic can thus be referred to as a “rational science” or what the Scholastics called “scientia rationalis”. Metaphysics in turn is concerned with real being and its attributes. In other words it is concerned with the very nature of a thing, with being itself, with the root principle and operations of existing things. Aristotle referred to it as philosophia prima, i.e. the first philosophy that deals with the most fundamental and deepest aspects of reality. Metaphysics (traditionally viewed as the queen of the sciences), while called a speculative philosophy and a general science, is informed by the findings of the special sciences as well as logic. Also, metaphysical arguments and views can only be successful if the overall metaphysical view is coherent and interprets the facts logically and consistently.
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Hopefully this will help future discussions about how PD should be structured as well as clear up misconceptions about the scope of philosophy and some of its sub-categories.