What is knowlege 2016 revision biconditionality, contingency, necessity, sufficiency
Or: Being very precise about
conditions for things
Today’s aim (s)
• To teach you some key terminology:
necessity, sufficiency, contingency, iff.
• To begin to think about definitions of
belief, truth, justification and
The Biconditional If(f)
• Sometimes the word ‘if’ only conveys a loose connection
between statements: “I will come with you to the pictures
if you go on Friday”. But I could go with you to the
pictures on another day.
• However, if I say, “I will come with you to the pictures if,
and only if, you go on Friday”, I am excluding other
possibilities (such as going on Tuesday).
• Philosophers call this if the ‘Biconditional If’ and spell it
‘iff’. Using ‘iff’ allows philosophers to specify conditions
for things more precisely.
If or Iff?
1. I will die if I stop breathing
2. I can make a hot cup of tea if I have hot
3. I will pass my exams if there is a miracle
4. If I eat any more I will be sick
Take the example of the
statement, “I will grow up to
be very fit if I exercise and
• Might it be possible to be
very fit without eating
sensibly and exercising?
• Are diet and exercise the
only things that have a
bearing on fitness?
Contingency, Sufficiency, Necessity
• If you can be very fit without a good diet and
plenty of exercise, then these conditions simply
aren’t necessary. They are contingent.
• But if, amongst other conditions, I must follow a
good diet and take exercise to be healthy, then
these conditions would be necessary.
• And if diet and exercise were the only conditions
that had to be met, then these conditions would
• Because for philosophers interested in analysing a
concept, only an exhaustive list of conditions for that
concept to exist is good enough.
• Philosophers like lists of conditions that are ‘individually
necessary and jointly sufficient’. Because then these
lists are really precise.
• Necessity (needed, but not enough for) isn’t the same as
sufficiency (the only thing(s) you need).
• It is usually easy to think of necessary conditions. But
very hard to think of sufficient ones.
Try: defining what conditions must obtain for your
smartphone to play music to your ears.
To sum up
• A condition is necessary when it must be the case for
something to be true. But other conditions could be
• A condition or set of conditions is sufficient when only
those things and nothing else must occur for something
to be true.
• Precisely specifying conditions allows for precise
• It can be very hard to specify sufficient sets of