The Social Scientist in me always strives to state the obvious. So here goes. I'm not at all certain I've heard this explained like this (directly) before, although the concept is omnipresent in the genealogical research process.
When setting out to answer a research question, we need to know these three things (Time, Place, Person) and look for the intersection where each cross the other. Think of a horizontal line representing Time, and a vertical line representing a Place. On your horizontal line, you can establish the date 25 November 1863. Intersect this with a place, say, Missionary Ridge Tennessee. If you happened to find yourself at Missionary Ridge Tennessee on 25 November 1863, you would have witnessed a deadly battle during the Civil War!
|Time, Place, Person(s)|
The Historian stops there. Time and Place = History. The Genealogist adds a person. Now think of the third line, intersecting the first two; a participant in the battle. Having these three items of
information the researcher can go forward and look for the information meeting the requirement of 1) 25 November 1863, 2) Missionary Ridge Tennessee, and 3) Colonel Augustus G. Tassin (Union, 35th Indiana).
Without any one of these three pieces of data, our efforts at research are lost. With each in mind, we can formulate a research strategy involving perhaps published works, government documents, etc.
This can be expanded to more than one person or even a family. In adoption and EPE cases, the intersection of these three is very necessary (think back to that awkward High School class for more information on the biology involved). Two persons in the same place at the same time. If one of the two can be shown to have been at another Place, at a given Time (like off to war for instance), then all lines do not intersect.
By the way, Time and Place can be dangerous in some situations (like driving a car). The retired policeman in me cautions against sharing Time and Place with fellow motorists!