Logic (Skt. तर्क Tarka; the science of logic is Nyāya or Tarkaśāstra) is the faculty and method of correct reasoning the activity of the intellect that checks whether the relations formed in the memory are coherent, consistent and true, rather than random and false. Where logical relations are the connections themselves, logic is the tool that tests and builds them.

On the path of knowledge, logic is one of the two pillars of knowledge the other being direct experience. A teaching is accepted or rejected only after it is checked against direct experience and logic.

Why logic matters

Experiences are continuously impinging on the memory. Left to themselves they would form a high-entropy, confused mass. The intellect arranges them into a low-entropy structure knowledge and logic is what keeps that structure consistent. Without logic we cannot distinguish knowledge from ignorance; we cannot predict, infer, or decide.

Reasoning in the Nyaya tradition

The classical science of logic (Nyāya) studies the valid and invalid forms of knowing. The main valid means (means of knowledge) are:

• Pratyakṣa perception, direct experience.

• Anumāna inference. From a sign (liṅga) we know another thing: where there is smoke, there is fire. The sign rests on a universal concomitance (vyāpti) wherever there is smoke, there is fire.

• Upamāna comparison / analogy. Knowledge arising from similarity between two things.

• Śabda testimony of a reliable, knowing source.

Invalid or incomplete apprehension (apramā) is of three kinds:

• Saṃśaya doubt, the oscillating state of mind holding mutually contradictory properties at once.

• Viparyaya perversion / illusion, knowing a thing as something else (a shell seen as silver).

• Tarka hypothetical or indirect reasoning, reductio ad absurdum: showing that denying a thesis leads to an undesired consequence.

Logic on the path of knowledge

Logic is used constantly on this path not to prove the experiencer (which is self-evident and beyond proof) but to clear away the false. When we ask what is the cause of experience? and follow the logic, we find that any assumed cause must itself be experienced, and so the chain never starts experience is causeless. When we ask how many experiencers are there? logic shows that to count many, they would have to be experienced, which is impossible. In this way logic dismantles ignorance and leaves the direct truth shining.

A caution

Logic is a tool, not a substitute for experience. Pure reasoning detached from direct experience produces only theories and abstractions intellectual knowledge that is really ignorance. Logic must always be grounded in and checked against your own direct experience.

See also: intellect, logical relations, means of knowledge, direct knowledge, criteria for truth, theory, ignorance.