ABSTRACTCurrent theories of probability recognise a distinction between external certainty and internal certainty. The present studies investigated this distinction in lay people's judgements of probability statements formulated to suggest either an internal or an external interpretation. These subtle differences in wording influenced participants' perceptions and endorsements of such statements, and their impressions of the speaker. External expressions were seen to signal more reliable task duration estimates, and a lower degree of external than internal certainty was deemed necessary to advise a (...) course of action. In conversations about football, internal expressions were perceived as signalling more personal interest, and were expected to be on the average 10% higher than corresponding external probabilities. Finally, people who reported their outcome expectations for two m... (shrink)