One frequent textbook definition of ‘knowledge’ is that it is a justified true belief. Each of the three parts of that definition can be understood in light of its negation: I can’t be said to know something if I don’t believe it, or if I lack any justification for that belief, or if the belief is untrue.
Linguistically, the English language refers to distinctly different types of knowledge with the same one word. This leads to confusions.
Compare ‘I know my friend Tom well’ with ‘I know that 37 times 7 is 259.’
Both proposition include the word ‘know,’ but it seems that the word does not refer to the same state. One is a familiarity or acquaintance with a person, place, or thing. The other is the possession of some mental content.
If there are different types of knowledge, then we might ask, what do they have in common, and what distinguishes them? Is truth a part of every different form of knowledge?
Truth seems to be explicitly or implicitly a part of most attempts to define ‘knowledge.’ If something is not true, then I usually cannot be said to know it. Propositions like ‘I know that 4 plus 9 is 37’ appear to us to be misuses of the word ‘know.’
It is difficult to conduct an investigation of knowledge without simultaneously conducting an investigation of truth. Just as there are competing understandings of knowledge, so there are competing understandings of truth.
A typical textbook explanation of knowledge relies on the concept of correspondence. According to this ‘correspondence theory of truth,’ a proposition is true if, and only if, it corresponds to the situation which it claims to represent.
One competing understanding of truth is represented by Martin Heidegger. He used the word ‘aletheia’ meaning ‘not hidden’ or ‘uncovered.’ Heidegger’s understanding of truth was not so much it corresponded to reality, but rather that it disclosed reality.
While Heidegger’s writings on truth are significant in the history of philosophy, and indicate a line of thought worth investigating, the mainstream of philosophy in Europe and North America during the twentieth century tended toward other understandings of truth. As a representative of British and analytical philosophies, the empiricist A.J. Ayer wrote:
I conclude then that the necessary and sufficient condition for knowing that something is the case are first that what one is said to know be true, secondly that one be sure of it, and thirdly that one should have the right to be sure. This right may be earned in various ways; but even if one could give a complete description of them it would be a mistake to try to build them into the definition of knowledge, just as it would be a mistake to try to incorporate our actual standards of goodness into a definition of good. And this being so, it turns out that the questions which philosophers raise about the possibility of knowledge are not all to be settled by discovering what knowledge is. For many of them reappear as questions about the legitimacy of the title to be sure. They need to be severally examined; and this is the main concern of what is called the theory of knowledge.
What Ayer calls the ‘theory of knowledge’ is perhaps more commonly known as epistemology. Just as it is difficult to discuss knowledge without also examining the notion of truth, so also questions about language invite themselves into the investigation.
Truth is generally understood to be a property of a proposition. Can a sentence, which is a linguistic artifact which represents a proposition, also be true?
One can know a proposition, and the proposition can be true, but it is a further step to know that a proposition is true. Presumably, one cannot know that a proposition is true without first having understood the proposition.
Someone can teach me to make a series of sounds, or a series of ink marks on paper, and tell me that they are a sentence which in turn represents a proposition, and that the proposition is true. I might take his word for it, and believe all this. Finally, he might be right, which would in turn make me right. But I could not be said to ‘know’ or to ‘understand’ the sentence or the proposition.
