Deduction, induction and abduction

1. Comparison

Type	Direction	Strength	Example
Deduction	general → particular	certain (given true premises)	All metals conduct → copper is a metal → copper conducts
Induction	particular → general	probable	Every bird seen flies → birds usually fly
Abduction	observation → cause	most likely (revisable)	The ground is wet → it probably rained

2. Strengths and weaknesses

Deduction is the most reliable, but it only reorganises information already contained in the premises; it produces no new knowledge, and false premises give false conclusions.

Induction produces new understanding but depends on the sample being representative — "all swans I have seen are white" held until a black swan appeared.

Abduction is closest to everyday judgement and is the engine of troubleshooting; the price is that it is only ever the current best explanation, which new evidence can rewrite.

3. Why troubleshooting starts with abduction

Diagnosis, debugging and detective work share one shape: there is a symptom, find the cause. That forces you to start with abduction — enumerate candidates, rank them by common sense and base rate, then design tests. Deduction can only check local consequences ("if it were a leak, there should be a stain nearby"); it cannot produce the cause on its own.

4. The most commonly confused sentence

"This explanation makes sense" describes the abductive plausibility of an idea — not deductive necessity, and not high probability. The three must be assessed separately.

5. Using them together

1. Abduction generates candidate explanations;
2. Occam's razor and base rates rank them;
3. Deduction derives the observable consequences of each, which become your tests;
4. Bayesian updating revises the probabilities as evidence arrives.

Related: abductive reasoning, Occam\'s razor, Bayesian thinking, method.