What issue related to the model’s disappearance does the Psychologist bring up for discussion?

The machine disappears. The men are astounded. The TT says he has nearly completed a larger machine, with which he intends to travel through time himself. When asked, he admits he does not know if the model has gone into the past or the future. The men reason that it would have been visible to them when they first came in (if sent to the past) or it would be visible now (if sent to the future). The TT asks the Psychologist to explain why this is not true; the Psychologist argues that the machine travels through time too quickly for them to appreciate it. The men are willing to believe this, at least for now. The TT shows them his larger machine, made of several metals and substances, in the his laboratory.