Following David Hume, many philosophers think that 'is' doesn't imply 'ought' (or that 'ought' cannot be derived from 'is'). In other words, normative statements about what ought to be the case cannot be validly derived from factual statements about what is the case. This is known as the "Is-Ought Gap."
If this is correct, then arguments, such as the following, are invalid:
Playing soccer is fun.
Therefore, I ought to play soccer.
But why can't an 'ought' be derived from an 'is'? Consider the following argument:
In situation X, Y is the right thing to do.
I am in situation X.
Therefore, I ought to Y.
Is this an instance of deriving 'ought' from 'is'? Is it invalid?