The law of uniformity of nature is considered as one of the formal grounds of induction.
The question is raised how we get the knowledge about this law as it is itself a case of inference. This problem is known as the problem of induction.
A similar problem is raised in the context of vyapti, which is considered as the ground of inference. The vyapti is the basis of reaching at the conclusion. But how do we ascertain vyapit?
To this important question, Naiyayikas, Buddhists and Vedantins have attempted to give certain logical justifications in the following manner.
a) Nyaya Standpoint
Naiyayikas have suggested that the invariable, unconditional, constant concomitant relation can be established through six ways, those are, namely, anvaya, vyatireka, vyabhicaragraha, upadhinirasa, tarka and samanya laksana pratyaksa.
Anvaya refers to the cases of experience of the positive instances where the constant concomitance between the two (vyapya and vyapaka) has been witnessed.
Here the knowledge is obtained through the knowledge of agreement in presence. It is just like Mill’s method of agreement that helps us in knowing the causal relation between two phenomena.
Once the causal relation between two phenomena is established it can be expressed in a general statement. Thus Vyapti can be ascertained through the experience of the positive instances of causal connections between two events.
Wherever smoke is seen their fire is also seen or wherever cloven hoofs are experienced there the presence of horns is also experienced.
ii) Vyatireka –
Vyatireka refers to such cases where the absence of one also decides the absence of the other phenomenon on a number of occasions. It is like the method of difference. It helps in arriving the causal connection. E.g. wherever fire is absent smoke is also absent.
iii) Vyabhicaragraha –
This step refers to the non-observation of contrary instances. It helps in strengthening the knowledge obtained through anvaya or vyatireka. If a single case of presence of the smoke and absence of the fire is not experienced it helps in reaching the general statement: wherever there is smoke there is fire.
This step puts emphasis on the relation of unconditionality. The sadhya and hetu are to be related unconditionally a condition is known as upadhi. The absence of the condition helps us in knowing the invariable relation between two phenomena.
The invariable relation leads to universal relation. Repeated observations or bhuyodarsana helps in arriving al the knowledge of unconditionality.
Smoke is produced when the fuel is found to be wet. This is not a condition to relate smoke with the fire. This is a condition to relate fire with the smoke.
All cases of fire are not smoky. Only those cases of fire, where the fuel is wet, are smoky. Thus smoke and fire are unconditionally related but not the fire and smoke. Because all cases of fire are not the cases of smoke.
In other words there can’t be smoke without fire, but there can be fire without having smokes. So smoke is unconditionally related with fire.
v) Tarka –
Tarka refers to hypothetical reasoning. The relation of concomitance should be supported properly by some reasoning (tarka). There are certain cases where reasoning helps us reaching general statements.
A generalization is established with more certainty through an indirect method of showing the impossibility of opposite tarka or reasoning. The opposite tarka would be some cases of smoke are not cases of fire. On this basis it is concluded that “All smoky objects are fiery”.
The causal connection between smoke and fire is accepted because we cannot assume that where smoke is present fire may be absent. Thus there is no contradiction in establishing causal connection between smoke and fire.
vi) Samanya laksara Pratyaksa –
In all the previous steps attempt has been made to establish causal Connection between two events basing on particular instances. But such a relation can also be confirmed looking to their class-characteristics. Naiyayikas have advocated in support of samanya laksana pratyaksa which is the perception of the universal element.
The class-character aims at the fundamental nature of the thing. The general characteristic of smoke is that it comes out of some fiery object. It can’t come out of non-fiery object.
Thus the smoke class and the fire class are related on the basis of which a universal statement can be established. Similarly out of the death of so many human beings, the perception of the causal connection between manhood and the mortality can be made. That is another example of samanya laksana pratyaksa.