The reasons for and general outline of Natural Philosophy are given in this entry and in the entries Natural Logic, Natural Realism and Rules of Reasoning:

Philosophy, so the Shorter Oxford English Dictionary on Historical Principles tells us is

1. (In the original and widest sense.) The love, study, or pursuit of wisdom, or of knowledge of things and their causes, whether theoretical or practical.
2. That more advanced study, to which, in the medieaval universities, the seven liberal arts were introductory; it included the three branches of natural, moral, and metaphysical philosophy, commonly called the three philosophies.
3. (= natural p.) The knowledge or study of natural objects and phenomena; now usu. called 'science'.
4. (= moral p.) The knowledge or study of the principles of human action or conduct; ethics.
5. (= metaphysical p.) That department of knowledge or study that deals with ultimate reality, or with the most general causes and principles of things. (Now the most usual sense.)
6. Occas. used esp. of knowledge obtained by natural reason, in contrast with revealed knowledge.
7. With of: The stude of the general principles of some particular branch of knowledge, experience or activity; also, less properly, of any subject or phenomenon.
8. A philosophical system or theory.
9. a. The system which a person forms for the conduct of life. b. The mental attitude or habit of a philosopher; serenity, resignation; calmness of temper.

This is as clear a definition as any, and I shall presume it for philosophy. It also immediately poses a problem we have to give some sort of initial answer to.

The fundamental problem of presuppositions

If we want to know or study "ultimate reality" (whatever that will turn out to be), what may we or may we not presuppose? This is a relevant question, if only because it seems that whatever we do presuppose will have some influence on whatever we come to conclude while also it seems we cannot conclude anything without presupposing something: To reach any conclusion one needs some assumption(s).

It is clear that any human philosophy is the product of people who already know and suppose something, in particular some Natural Language to reason and communicate with. So any human being concerned with philosophy uses and presumes in some sense some Natural Language.

Natural Language

Hence we start with presuming some Natural Language

• consisting of words and statements (both sequences of letters) that enable its speakers to represent things to themselves and to other speakers by pronouncing or writing down the words or statements that represent those things

• in which, at least initially, we can frame philosophical questions and provide philosophical answers, where we take "philosophy" in the sense just given, or in brief as: The search for rationally tenable explanations for all manner of things;

• and it is also clear that each and every human being that speaks a natural language therewith has a means to claim about any of its statements that it is true or not, credible or not, necessary or not, and much more ("probable", "plausible", "politically correct", "sexist", "morally desirable" a.s.o.)

For the purpose of doing philosophy, in the sense of seriously attempting to ask and answer general questions, some natural language must be considered given, for without it there simply are no questions to pose or answer. And indeed, all philosophy, including any philosophy that concludes there is no human knowledge, in fact presumes some natural language.

This is itself a fact of some philosophical importance that is often disregarded. One of its important applications is to show that people who propound skeptical arguments to the effect that human beings cannot know anything, or cannot know anything with certainty, or cannot know anything with more or less probability than its denial (these are three somewhat different versions of skepticism, that also has other variants that are less easy to refute) must be mistaken, since thy all presuppose some natural language known well enough to state claims that nothing can be known.

It should als be noted with some care that a natural language is not given to human beings in a completely clear, perfect and obvious way (since, for example, it is very difficult to clearly articulate the rules of grammar one does use automatically and correctly when speaking it), but it is given to start with as a tool for communication and expression that may be improved and questioned, and that enables one to pose and answer questions of any kind.

Natural language is, in other and somewhat technical words, a heuristic, i.e. something that helps one find out things. What other heuristics do come with being human? Every Natural Language includes many terms and many - usually not very explicit and articulated - rules that enable its users to represent their experiences, and to reason or argue with themselves or others. We shall call this body of terms and rules Natural Logic.