Knowledge or cognition (jriana or buddhi) is the manifestation of objects. Just as the light of a lamp reveals or shows physical things, so knowledge manifests all its objects.
Knowledge is broadly divided into anubhava or presentative cognition and smrti or memory, i.e., representative cognition. Each of the two can be valid (yathartha) or non-valid (ayathartha).
Valid preventative knowledge is called prama. It is divided into perception, inference, comparison and testimony. Non-valid presentative knowledge (aprarna) is divided into doubt (samsaya), error (bhrama or viparyyaya) and hypothetical argument (tarka).
Thus valid presentative knowledge (prama) is a definite or certain (asandigdha), faithful or unerring (yathartha), and non- reproductive experience (anubhava) of the object.
My visual perception of the table before me is such knowledge (prama) because in it the table is presented to me directly just as it really is and I am certain about the truth of my cognition.
Though memory is not prama, asit is non-preventative or a mere reproduction of past knowledge, it may also be valid or non-valid, according as it is a reproduction of some previous valid or non-valid preventative knowledge.
Doubtful cognition cannot be called prama, because it is not certain knowledge. Error is undoubted knowledge indeed, and may also be preventative, but it is not true to the nature of its object.
Sometimes we perceive a snake in a rope in the twilight and have then no doubt about the reality of what we see. Still this perception is erroneous, because it is not a true cognition of the object (yatharthanubhava).
Tarka is not prama, since it does not give us any knowledge of objects. A tarka is like this: Looking out of the window of your classroom you see a mass of smoke rising from a distant house and say that the house has caught fire.
A friend contradicts you and asserts that there is no fire. Now you argue: If there is no fire, there cannot be smoke. This argument, starting with an ‘if,’ and exposing the absurdity of your friend’s position, and thereby indirectly proving your own, is tarka.
It is not prama or valid presentative knowledge, because to argue like this is not to know the fire, but to confirm} our previous inference of fire from smoke. That there is fire, you know by inference.
To argue that if there is no fire there Vide Tarkasangraha, p. 84. Some Mimamsakas also exclude memory from prama, on the ground that it does not give us any new knowledge.
It is only a reproduction of some past experience and not cognition of anything not known before (anadhigata). Cannot be smoke, is not to know the fire as a real fact either by way of perception or by that of inference.
The next question is: How is true knowledge distinguished from false knowledge? Knowledge is true when it agrees with or corresponds to the nature of its object, otherwise it becomes false.
Your knowledge of the rose as red is true, if the rose has really a red colour as you judge it to have (tadvati tatprakaraka).
On the contrary, your impression of the crow as white is false, since the white colour does not really belong to the crow; the white colour is ascribed to the crow in which it is absent (tadabhavavati tatprakaraka).
But then it may be asked: How do we know that the first knowledge is true and the second false?
In other words: How do we test the truth or falsity of knowledge? The Naiyayikas (also the Vaisesikas, Jainas and Bauddhas) explain it in the following manner: Suppose you want a little more sugar for your morning tea and take a spoonful of it from the cup before you and put it into your tea.
Now the tea tastes sweeter than before and you know that your previous perception of sugar was true.
Sometimes, however, it happens that while looking for sugar, you find some white powdered substance and put a pinch of it into your mouth under the impression that it is sugar.
But to your utter surprise and disappointment, you find that it is salt and not sugar. Here then we see that the truth and falsity of knowledge consist respectively in its correspondence and non-correspondence to facts.
But the test of its truth or falsity consists in inference from the success or failure of our practical activities in relation to its object (pravrttisamarthya or pravrttivisamvada). True knowledge leads to successful practical activity, while false knowledge ends in failure and disappointment.
Western logic, the problem of perception as a source of knowledge has not been properly discussed. The reason probably is this. We generally believe that what is given in perception must be true.
Ordinarily, no man questions the truth of what he perceives by his senses. So it is thought that it is unnecessary, if not ridiculous, to examine the validity of perception, or to determine the conditions of perception as a source of valid knowledge.
Indian thinkers are more critical than dogmatic in this respect, and make a thorough examination of perception in almost the same way as Western logicians discuss the problem of inference.