If you still are not clear about Sufficient and Necessary conditions, please read the following article, which I found helpful.

The Concepts of

Necessary and Sufficient Conditions

Contents

1. Introduction
   
2. Definition of "necessary condition"
   
3. Definition of "sufficient condition"
   
4. Necessary conditions that are not jointly sufficient
   
5. Sufficient conditions that are not necessary
   
6. The concept of converse relations
   
7. "Is a necessary condition for" and "is a sufficient condition for" are converse relations
   
8. Four possible combinations

• Practice exercise #1
  
• Practice exercise #2

9. Different kinds (or modes) of necessary condition

• "Air is necessary for human life."
  
• "Human beings must have air to live."
  
• "Without air, human beings die (i.e. do not live)."
  
• "If a human being is alive, then that human being has air (to breathe)."

• "Having a microscope (or some other instrument) is a necessary condition for (our) seeing viruses."
  
• "A microscope (or a similar instrument) is needed to see viruses."
  
• "Human beings must use (have) a microscope to see viruses."
  
• "Human beings cannot see viruses without a microscope."
  
• "Anyone who sees viruses has (uses) a microscope."
  
• "If someone sees viruses, then that person uses a microscope." NOTE
  
• "Without a microscope, a person cannot see viruses."
  
• "If a person does not have (the use of) a microscope, then that person does not see viruses."
  
• "Whoever lacks a microscope does not see viruses."
  
• "One must have a microscope to see viruses."

Example 4.1 - A set of conditions that are individually necessary without being jointly sufficient.

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Thomas White, the author of a recent textbook in philosophy, attempted to use as his example the specifying of the necessary and sufficient conditions for hearing music from a Walkman®. Here is the list of necessary conditions that White offered (irrelevant conditions have been here removed, and the list has been renumbered):

1. The Walkman is in good working order.
   
2. The batteries are good. [ Note 1 ]
   
3. The earphones are plugged in.
   
4. The tape has music on it and is in good working condition.
   
5. You operate the controls correctly.
   
   
   -- Discovering Philosophy, by Thomas I. White, p. 25 (Englewood Cliffs, NJ: Prentice Hall), 1991.

White then goes on to make a too-hasty claim: "... taken all together they are sufficient " (ibid., p. 25). Unfortunately, for his illustrative purposes, the list is by no means sufficient. Here are just a few of the many additional necessary conditions that my own students, in previous years, have offered:

6. The listener must not be deaf.
   
7. The ambient (surrounding) sound must not drown out the earphones.
   
8. The listener must be wearing the earphones, or must be close enough to them, to hear the music.
   
9. There must be nothing blocking the sound in the listener's ears.
   
10. The tape must be inserted correctly; the door of the Walkman must be closed; and the tape must not be at the end of the reel (more specifically, it must be positioned so that some of the parts of the tape which contain recorded music will pass over the playback heads).
    
11. The earphones are in good working order.
    
12. The listener does not die in the time between operating the controls correctly and the music's emerging from the earphones.

Even this expanded list is not complete. I imagine that will little effort, you yourself can come up with still further necessary conditions. Indeed, there does not seem to be any practical limit to the number of necessary conditions.

Example 5.2 - A(nother) set of conditions that are (jointly) sufficient without being individually necessary.

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If you'll forgive the morbid example, think of all the ways a person might die: having his/her head chopped off; being at `Ground-Zero' when a nuclear bomb is detonated; tearing a gaping hole in one's space suit while on a `space-walk' on the Moon; etc., etc. But none of these conditions is a necessary condition for a person's dying. Indeed almost everyone dies without having satisfied one of these unusual sufficient conditions.

• ... is a daughter of ...
  
• ... is a parent of ...

• ... is not taller than ...
  
• ... is taller than ...

• "Since having a microscope is necessary for seeing viruses, then seeing viruses guarantees that one has a microscope (i.e. suffices for having a microscope)."
  
  
  
• Similarly, "Since having air to breathe is necessary for human life, if follows that the existence of human life (demonstrates, assures, guarantees, i.e.) suffices for the existence of air."
  
  
  
• "Since every square must have four sides (i.e. since having four sides is a necessary condition for being a square), being a square is a sufficient condition (but not a necessary one) for something's having four sides." Put another way: "All squares (must) have four sides; but not all four-sided things [e.g. trapezoids] are squares."
  
  
  
• "Being a father is a sufficient condition for being male, and being male is a necessary condition for being a father." (But being a father is not a necessary condition for being a male [e.g. James Dean was not a father, but was male]; and being a male is not a sufficient condition for being a father [again the case of James Dean].)

• "Sam's being a male is a necessary, but not a sufficient condition, for being a father."
  
• "A table's having four sides is a necessary, but not a sufficient, condition for its being square."
  
• "Being more than 6 feet (183 centimeters) tall is a necessary, but not a sufficient condition for being 6 feet 3 inches (190.5 centimeters) tall."
  
• "Owning a 1996 Chevrolet Cavalier is a necessary, but not a sufficient condition, for owning a red, 6-cylinder, 1996 Chevrolet Cavalier."
  
• "Having a ticket in a lottery is a necessary, but not a sufficient condition, for winning that lottery."

• "Sam's being a father is a sufficient, but not a necessary condition, for being a male."
  
• "A table's being square is a sufficient, but not a necessary condition, for its having four sides."
  
• "Being 6 feet 3 inches tall (190.5 centimeters) tall is a sufficient, but not a necessary condition for being more than 6 feet (180 centimeters) tall."
  
• "Owning a red, 6-cylinder, 1996 Chevrolet Cavalier is a sufficient, but not a necessary condition, for owning a 1996 Chevrolet Cavalier."
  
• "Winning a lottery is a sufficient, but not a necessary condition, for having a ticket."

• "Sam's being a father is both a necessary and a sufficient condition for his being a male parent."
  
• "Frankie's being older than Johnny is both a necessary and a sufficient condition for Johnny's being younger than Frankie."
  
• "Today's being neither Saturday nor Sunday is both a necessary and a sufficient condition for today's being a weekday."
  
• [TRICKY, but true.] "X's-being-a-necessary-condition-for-Y is both a necessary and a sufficient condition for Y's-being-a-sufficient-condition-for-X."

• "John's loving Pamela is neither a necessary nor a sufficient condition for Pamela's loving John."
  
• "Having a married brother is neither a necessary nor a sufficient condition for being an aunt."
  
• "Being the smartest student in a class is neither a necessary nor a sufficient condition for achieving the highest grade in that class."
  
• "Aziz's playing baseball well is neither a necessary nor a sufficient condition for his having a driver's license."
  
• "Wanting to succeed is neither a necessary nor a sufficient condition for success."

NOTES

1. Strictly, although the batteries' being good is usually a necessary condition for a Walkman's working, it is not absolutely necessary. For it is possible to operate a Walkman without batteries, e.g. by using a plug-in AC-to-DC adapter. What is necessary, along these lines, is that there be a proper power source for the Walkman.