Logic | Concept & Branches of Logic

Logic is the examination of the principles and methods used to separate correct reasoning from incorrect reasoning. Logic is also a branch of philosophy that studies rules and formulae that help us to distinguish between bad and good argument. Logic helps to answer such questions as: what is correct reasoning? What distinguish a good argument from a bad one? Are there methods to detect fallacies in reasoning, and if so, what are they?

Concept of Logic

Logic is both an art and science, and it is on the one hand, a rigorous academic or theoretical discipline and on the other hand it is practically applicable field that can be used in day to day activities.

Therefore, logic brings about correct and sound reasoning. The application of the systems and techniques of logic to real life situation directs us into the area of applied logic. Logic deals with reasoning as features of thinking. Logic differs from psychology in that it does not deal with all types of thinking, such as learning, remembering, day –dreaming, supposing and so forth, but only with that type of thinking called reasoning.

We must note that all reasoning is thinking but not all thinking is reasoning.Because one can imagine himself to be a pilot; you may remember something you should have done two days ago or I may regret an action I performed or I may be even be day dreaming. All these are not reasoning but they are definitely thinking.

Branches of Logic

  • Formal and informal logic
  • Symbolic logic
  • Logical theory
  • Traditional logic
  • Applied logic
  • Deductive and inductive logic
Technical terms in Logic
  • Proposition: this is a technical term for a meaningful claim, statement, sentence which affirms or denies something. In logic proposition are taken to be either true or false. For instance, when I hold that my father is the king. This statement can either be true or false after we have investigated the truth or falsity of the claim it makes.

However, there are some statements that can neither be true nor false. For example commands like: stop there! Keep quiet! Or exclamations like Oh my God! Other statement that can neither be true nor false includes prayer and requests. It is possible that two sentence with different composition can express identical meaning. When this happens, both sentences are said to affirm the same proposition for example: Richard beats Harry. Harry is beaten by Richard.

  • Inference: this is a process by which we make a claim on the basis of the claim made in other propositions. That is a system of deriving and affirming a proposition. For example, we can derive a third proposition from the two given below.

“Anybody who was in the speedboat and who wore military outfit was a terrorist”

Osama was in the speedboat and wore a military outfit, therefore, Osama was a terrorist.

In inference, you start with one or more propositions which have been accepted; you then use those propositions to arrive at a new proposition.

If the inference is valid, that proposition should be accepted.You can use the new proposition for interference later on.

So initially, you can only infer things from the premises of the argument. But as the argument proceeds, the number of statements available for inference increases.

  • Argument: an argument is a piece of discourse in which a claim is made and reasons are given in support of the truth of the claim. That is, an argument makes a point and tries to justify that point. An argument has two parts, the premise and the conclusion. An argument is a set of statements or proposition one of which is claimed to be true o the basis of the others.

Example of argument: all men are animals, Mr. Harry is a man, and therefore, Mr. Harry is an animal. An argument is a group of propositions of which one is claimed to follow from the others which are regarded as providing support or ground for the truth of that one.

Note that you can build a valid argument from false premises and arrive at a false conclusion. The tricky part is that you can start with false premises, proceed via valid inference, and reach a true conclusion.


Premises: all birds live in a nest.

Premises: chickens are birds

Conclusion: therefore chicken live in a nest.

  • Premises: premises are the reasons or ground given or offered for accepting the conclusion that is claimed. Consider the argument below.

All spiders have six legs

All six-legged creatures have wings

Therefore, all spiders have wings.

  • Conclusion: this is the claim made on the basis of the other propositions in an argument. In other words, the proposition that one is trying to prove or establish is called the conclusion. Note that when the premises of an argument are true, it will be impossible for the conclusion to be false. However, there can be a situation where the premises are true and the conclusion is false. The conclusion is the result of the final step of inference. It is only a conclusion in the context of a particular argument; it could be a premise or assumption in another argument. The conclusion is said to be affirmed on the basis of the premises and the inference from them.
  • Syllogism: this means an argument that contains two premises and at least a conclusion.
  • Indicators: these are certain words or phrases that show us either the premises or the conclusion of an argument.

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