Seeing things

People see different things - ever if they are ostensibly looking at the same thing: that is apparently why the Police prefer to have one witness only to an event. rather than several. We've all heard of experiments were psychology students watch a video and are told to watch for something - and are so busy concentrating on that, they don't notice the gorilla. We all have selective attention - and filtering! Hopefully I will never be called on to be a witness. I know my powers of observation and/or recall are, well, challenged.