Difference Between Necessary & Sufficient Conditions

Although the words Necessary and Sufficient are two words that are often used interchangeably in the English language, there is a clear difference between them. We use these two words when referring to a relationship between two things. Let us comprehend the difference between the two in the following manner. If we say that A is necessary for the existence of B, it highlights that A is a mandatory condition that needs to be met for B to exist. On the other hand, in the sufficient condition, it highlights that A’s existence guarantees B’s existence as well. Simply, if A does not exist neither can B. This highlights that a subtle difference exists between necessary and sufficient.

Key Takeaways

• Necessary refers to a mandatory condition that must be met for something else to exist.
• Sufficient refers to the minimum requirement that guarantees the existence of something else.
• Necessary is used with a sense of definiteness, while sufficient is used with a sense of indefiniteness.

What is Necessary?

The word ‘necessary’ is used in the sense of something that is essential for the completion of a work, a concept or an action. This is to say that it is mandatory to have a specific condition for another to exist.  Look at the following sentences:

Water is necessary for the survival of man.
A passport-sized photograph is also necessary.
It is necessary to fill up all the particulars in the application form.

In the first sentence, you get the idea that water is very much required or essential for the very survival of man. It also highlights that not having water is linked to man’s inability to survive. Hence, water becomes a mandatory condition that needs to be met for man’s survival.  In the second sentence, you get the idea that it is essential to submit a passport-sized photograph. In the third sentence, you get the idea that it is essential or required that you should fill up all the particulars included in the application form.

What is Sufficient?

The word ‘sufficient’ is used in the sense of ‘what is enough’. It gives the additional sense of ‘minimum requirement’. It highlights that a particular condition being in existence guarantees that the other condition also exists.
Observe the following sentences:

It is sufficient if you carry 50 dollars.
The flask has sufficient water in it.

In both the sentences, you get the idea of the minimum requirement. In the first sentence, you get the idea of the minimum requirement of 50 dollars to buy something. In the second sentence, you get the idea of the minimum requirement of water to satisfy your thirst or to gulp a tablet of medication.
Another important difference between the two words, ‘necessary’ and ‘sufficient’ is that the former is used in the sense of definiteness while the latter is used in the sense of indefiniteness. In other words it can be said that there is definiteness about what is required in the case of the usage of the word ‘necessary’, whereas there is indefiniteness about what is required in the case of the usage of the word ‘sufficient’ as in the sentence ‘I think the water in the overhead tank is sufficient for the day’.
In the sentence given above the speaker is not sure about the quantity of water present in the overhead tank and he is also not sure whether it is enough for the day. This kind of doubt does not exist in the case of the use of the word ‘necessary’. Hence, you will have to be very careful while using the two words ‘necessary’ and ‘sufficient’ so that they convey their meanings fully well.

What is the Difference Between Necessary and Sufficient?

Definitions of Necessary and Sufficient:
Necessary: If we say that A is necessary for the existence of B, it highlights that A is a mandatory condition that needs to be met for B to exist.
Sufficient: In the sufficient condition, it highlights that A’s existence guarantees B’s existence as well.
Characteristics of Necessary and Sufficient:
Requirement:
Necessary: The word ‘necessary’ is used in the sense of ‘absolute requirement’.
Sufficient: It gives the additional sense of ‘minimum requirement’.
Definiteness:
Necessary: Necessary is used in the sense of definiteness.
Sufficient: Sufficient is used in the sense of indefiniteness.