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 mediaeval 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 study 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 we shall presume it for our subject. It also immediately poses a problem we have to give some sort of initial answer to. Up
The fundamental problem of presuppositions
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: Up
So it seems we must 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,
§ 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 true, 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 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 philosophies that conclude 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 they all presuppose some natural language known well enough to state claims that nothing can be known.
It should also 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: Up
§ a collection of terms and rules that come with Natural Language that allows us to reason and argue in it.
Examples of such logical terms are: "and", "or", "not", "true", "false", "if", "therefore", "every", "some", "necessary, "possible" and many more. Examples of such logical rules, that are here formulated in terms of what one may write down on the strength of what one already has written down (pretending for the moment that natural language is written rather than spoken) are: "If one has written down that if one statement is true then another statement is true, and if one has written down that the one statement is true, then one may write down (in conclusion) that the other statement is true" (thus: "if it rains then it gets wet and it rains, therefore it gets wet") and "If one has written down that every so-and-so is such-and-such, and this is a so-and-so, then one may write down that this is a such-and-such" (thus: "if every Greek is human and Socrates is a Greek, therefore Socrates is human").
We presuppose Natural Logic in much the same way as we presuppose Natural Language: as something we have to start with and precisify later, and that may well come to be revised or extended quite seriously, but also as something that at least seems to be in part given in more or less the same way to any able speaker of a Natural Language: In it there are a considerable number of terms and - usually implicit - rules which enable every speaker of the language to argue and reason, that every speaker knows and has extensive experience with.
Again, it does not follow that these rules and terms are clear or sacrosanct. All that I assume is that they come with Natural Language and are to some extent articulated in Natural Language and understood and presupposed by everyone who uses Natural Language.
Three very fundamental assumptions about the making of assumptions that come with Natural Logic are as follows - where it should be noted I am not stating these assumptions with more precision than may be supposed here and now:
1. Nothing can be argued without the making of assumptions
2. An assumption is a statement that is supposed to be true
3. Human beings are free to assume whatever they please
These I suppose to be true statements about arguments and people arguing, where it should be noted that especially the third assumption, factually correct though it seems to be, has been widely denied in human history for political, religious or philosophical reasons: In most places, at most times, people have not been allowed to speak publicly about all assumptions they can make.
Three other assumptions about argumentation that should be mentioned here are
1. Conclusions are statements that are inferred in arguments from earlier assumptions and conclusions by means of assumptions called rules of inference, that state which kinds of statements may be concluded from the assumption of which kinds of statements
2. Definitions of terms are assumptions to the effect that a certain term may be substituted by a certain other term in a certain kind of arguments
3. Rational argumentation about a topic starts with explicating rules of inference, assumptions and definitions of terms, and proceeds with the adding of conclusions only if these do follow by some assumed rule of inference.
The first two assumptions need more clarification than will be given here and now, but, on the other hand, again every speaker of a Natural Language will have some understanding of setting up arguments in terms of assumptions, definitions and rules of inference, and drawing conclusions from these assumptions and definitions by means of these rules of inference.
The third assumption, when compared with the normal practice of people arguing, entails that mostly people do not argue very rationally, at least in the sense that all too often they rely in their arguments on rules of inference, assumptions or definitions they have not explicitly assumed yet have used in the course of the argument.
Next, it seems that most users of most natural languages presuppose a metaphysics I shall call Natural Realism. This may be stated in many ways, for example in terms of the following assumptions: Up
§ There is one reality that exists apart from what human beings think and feel about it.
§ This reality is made up of kinds of things which have properties and stand in relations.
§ Human beings form part of that reality and have experiences about it that originate in it.
§ All living human beings have beliefs and desires about many real and unreal entities, that are about what they think is the case in reality and should be the case in reality.
§ All living human beings have very similar or identical feelings, sensations and beliefs and desires in many ordinary similar or identical circumstances.
What this might mean precisely, especially what may be meant by the bold terms, will concern us later.
However, the present point is that some assumption like natural realism is at the basis of human social interaction, at the basis of the law, and at the basis of promises, contracts and agreements.
We shall assume it is also at the basis of philosophy, at least initially, firstly, because we must assume something to conclude anything; secondly, because even if we - now or eventually - disagree with Natural Realism it helps to try to state clearly what it amounts to; and thirdly, because it does seem assumptions like those of Natural Realism are involved in much human reasoning about themselves and others, and about language, meaning and reality.
I shall presuppose Natural Language, Natural Realism and Natural Logic in what follows, and shall repeatedly turn to the issues what these assumptions all might mean in more precise (but at least equally intuitive and clear) terms and whether they are true, and if so, in what sense. Up
This is a somewhat reformatted part of my General Introduction to Leibniz's "Nouveaux Essays" in my section Philosopy.