Observing a system causes its wave functions to change indeterministically, because "the entering of an impression into our consciousness" implies a revision of "the probabilities for different impressions which we expect to receive in the future".
OBseRvInG a SysTem causeS its WAVe FunctionS to ChanGE InDETERminiSTicallY, becaUse "The entering of an iMPReSsion intO OUr consciousnEsS" impLIes A rEviSIOn Of "the proBAbilitIes FoR dIffeReNt ImPrEssIons wHicH we exPect to receiVe in The fuTURE".