A perceptible physical event, like a flash or a noise, may therefore be defined as a certain group of sensa having certain similarities to each other and certain neutral spatial relations. The neutral dates of the sensa in such a group are none of them earlier than the date of a certain physical movement, such as the opening of a shutter. Actually only a few of the sensa in such groups are sensed by anyone, and it may quite well happen that only one of them is sensed. The perceptual judgment does not assert that other sensa of the group are sensed, but only that they would be by any observer sufficiently like ourselves placed in any suitable position. Scientific Time is conceived by analogy with a sense-history. The scientific dates of sensa would seem to be the dates at which certain scientific events happen in the brain of the observer who senses these sensa.