Step 20 (following
Step 19).
Now to conclude this brief abstract about coming to validly know for oneself by means of buddhist logic there still remains the topic of categories of correct signs.
There are three 3 main categories of correct sigs:
1. Effect signs
2. Nature signs
3. Nonobservation signs
As to 1:
A correct effect sign is the three modes and the predicate of the syllogism is the sign's cause. So the sign is the effect which proves its cause. That means that predicate and sign are different entities.
Please note that the reverse is not valid: it is not possible to prove an effect by its cause as sign. Why? Because a potential cause always may be obstructed.
As to 2:
A correct nature sign is the three modes and the subject of the syllogism is both, the predicate and the sign. That means that predicate and sign are of one entity, i.e. two different properties of the subject.
As to 3:
A correct nonobservation sign is the three modes and the thesis of the syllogism is based on a
negation of a predicate, e.g. 'is not {the predicate}' or '{the predicate} does not exist in ...' or '{the subject} is empty of {the predicate}' and the like.
There may be a several relationships between the nonobservation signs and the negative predicate:
3.1 Nonobservation of a Cause
Where a specific cause cannot be observed its effect must be absent too.
3.2. Nonobservation of a Pervader
Where a certain type of phenomena that contains the predicate cannot be observed the predicate must be absent too. E.g. where there are not trees, there are no oaks.
3.3 Nonobservation of a Nature
Where there is no valid perception of an object that object is absent. This is because perception of an object and existence of that object are equivalent in nature.
3.4 Correct Signs That Are an Observation of an Object That Is Contradictory in the Sense of Not Abiding Together
E.g. cold and fire do not abide together.
3.5 Correct Signs That Are an Observation of an Object That Is Contradictory in the Sense of Mutual Exclusion
I.e. although the objects may abide together they are mutually exclusive. E.g. although a car and a tree may abide together that which displays the characteristics of a car is not a tree.
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If you are interested in more details about the topic please be referred to the literature mentioned.