Full Idea
If a sentence is effectively undecidable, the condition which must obtain for it to be true is not one which we are capable of recognising whenever it obtains, or of getting ourselves in a position to do so.
Gist of Idea
If a sentence is effectively undecidable, we can never know its truth conditions
Source
Michael Dummett (The philosophical basis of intuitionist logic [1973], p.225)
Book Reference
Dummett,Michael: 'Truth and Other Enigmas' [Duckworth 1978], p.225
A Reaction
The instances of 'undecidable' sentences are most clearly seen in mathematics, such as the Continuum Hypothesis or Goldbach's Conjecture, or anything involving vast infinite cardinals. But do you need precise truth-conditions for meaning?