A conclusion is sound (true) or unsound (false), depending on the truth of the original premises (for any premise may be true or false).
At the same time, independent of the truth or falsity of the premises, the deductive inference itself (the process of "connecting the dots" from premise to conclusion) is either valid or invalid. The inferential process can be valid even if the premise is false:
*There is no such thing as drought in the West.
California is in the West.
California need never make plans to deal with a drought*.
In the example above, though the inferential process itself is valid, the conclusion is false because the premise, There is no such thing as drought in the West, is false.
A syllogism yields a false conclusion if either of its propositions is false. A syllogism like this is particularly insidious because it looks so very logical–it is, in fact, logical.
But whether in error or malice, if either of the propositions above is wrong, then a policy decision based upon it (California need never make plans to deal with a drought) probably would fail to serve the public interest.
Assuming the propositions are sound, the rather stern logic of deductive reasoning can give you absolutely certain conclusions.
However, deductive reasoning cannot really increase human knowledge (it is nonampliative) because the conclusions yielded by deductive reasoning are tautologies - statements that are contained within the premises and virtually self-evident.
Therefore, while with the deductive reasoning we can make observations and expand implications, we cannot make predictions about future or otherwise non-observed phenomena.
Let us juxtapose the deductive reasoning process with "Inductive Reasoning"
Inductive reasoning begins with observations that are specific and limited in scope, and proceeds to a generalized conclusion that is likely, but not certain, in light of accumulated evidence.
one could say that inductive reasoning moves from the specific to the general.
Much scientific research is carried out by the inductive method: gathering evidence, seeking patterns, and forming a hypothesis or theory to explain what is seen.
Conclusions reached by the inductive method are not logical necessities; no amount of inductive evidence guarantees the conclusion.
This is because there is no way to know that all the possible evidence has been gathered, and that there exists no further bit of unobserved evidence that might invalidate my hypothesis.
Thus, while the newspapers might report the conclusions of scientific research as absolutes, scientific literature itself uses more cautious language, the language of inductively reached, probable conclusions:
Because inductive conclusions are not logical necessities, inductive arguments are not simply true. Rather, they are cogent:
that is, the evidence seems complete, relevant, and generally convincing, and the conclusion is therefore probably true. Nor are inductive arguments simply false; rather, they are not cogent.
It is an important difference from deductive reasoning that, while inductive reasoning cannot yield an absolutely certain conclusion, it can actually increase human knowledge (it is ampliative). It can make predictions about future events or as-yet unobserved phenomena.
Albert Einstein observed the movement of a pocket compass when he was five years old and became fascinated with the idea that something invisible in the space around the compass needle was causing it to move.
This observation, combined with additional observations (of moving trains, for example) and the results of logical and mathematical tools (deduction), resulted in a rule that fit his observations and could predict events that were as yet unobserved.
Thereby the deductive process was used as an additional tool, (when it can be used efficiently) but the new knowledge were constructed from a process of induction...