The Intensional operator «savoir» (to know). It is undertaken to explain that the following forms * Je ne sais pas que p (I do not know that p) * Je sais si p (I know if p), which present difficulties of reception, may however be used under certain conditions, notably in connection with the verb «dire» (to say): Je ne sais pas que p = «Je dirai/tu diras que je ne sais pas que p») (1 do not know that p = «I will say /You will say that I do not know that p») Je sais si p = «Je le sais, mais je ne le dirai pas» (I know if p = «I know it, but I will not say it»). The notion which is used is that of the universe of belief, and it is attempted to demonstrate that it is more advantageous in linguistics than a plurality of epistemic worlds. The difference between (ne pas) savoir/ dire que p «not) to know/ to say that p) and (ne pas) savoir/ dire si p «not) to know/ to say if p) lies essentially in the fact that the conjunction que (that) signifies the explication of p in at least one universe of belief (either taken into account or rejected) whereas the indirect interrogative si (if) leaves the value of p inexplicated.