For what it’s worth.

Scientist is to test subject as observer is to...

But wait, scientists are observers - of the natural world. Science is based off of what is seen, and they use their observations to help understand what we couldn't before. The different between scientists and observers, other than perhaps a Ph.D, is that scientist use test subjects, at times, to understand the cause and effect relationship between two individuals. Often you will have a control variable, which is untouched, and then various test subjects that have slightly altered variables, for comparison.

Observers don't do that. They just sit and look. With wide eyes and in some sort of crouch or even on their stomachs or backs- whatever it takes to get the best view. They look at things as they are, and do not interfere whatsoever with the natural or observed realm, for they are just visitors.

The scientist and the average observer. One is active, one is passive. One is engaged, one is an onlooker. I would say, therefore, that Scientist is to test subject as observer is to a guest or visitor. Because we have no greater privilege than to explore and investigate the realm in which we dwell, as respectful, quiet observers - as honored guests.