Let me give an example. Here is a screenshot of one of my ancestor's Detail page from FamilySearch.org Family Tree:
This is an entry for Samuel Linton supposedly born in 1775 in Derry, Ireland. There is no information entered as to a Christening date, Death date or any burial information. If I scroll down, I see that there are no sources listed. Here is another screenshot showing the Sources field:
How do we know if the birth date is correct? How do we know this is the correct "Samuel Linton?"
Where did the information about his birth date come from? Where would I go to verify that the person who entered the information originally selected the right person or even copied the information correctly? For example, if I go to Ancestry.com and search for "Samuel Linton," I get 561,418 returns. Which of these is my Samuel Linton? If I add in the date and supposed place of birth, I reduce that number to 55,824 and none of the entries has a resemblance to the Samuel Linton listed in the Family Tree. Here is one entry that might merit some additional investigation in there were any way to do that?
Samuel Linton in the Ireland, Griffith’s Valuation, 1847-1864The only problem with this entry is that my Samuel Linton would have been 72 years old (assuming the birthdate was correct) at the time of the earliest entries in this record. In addition, I find that my Samuel Linton is supposed to be the father of William Linton born about 1799 in Ardstraw, Tyrone, Ireland. My conclusion, the Samuel Linton in the Ancestry.com record cannot be the same as my own Samuel Linton.
OCCUPIER: Samuel Linton
LESSOR: Worshipful Company of Ironmongers
OS PAGE NUMBERS: 26
Essentially, I am going about this all wrong. Instead of now searching for someone with no sources, I should be looking forward in time to his children and making sure that I have the right people listed in the Family Tree. If I look at the person listed in my line who is supposed to be the son of the 1775 Samuel Linton, then I find that the William Linton has sources but none of them substantiate the birth information I noted above.
Over the years, genealogists and family historians have discussed the issues of "evidence" and "proof." All of this is meaningless until there are sources recorded that substantiate the information. The whole process of determining the correct conclusions from the "evidence" presupposes that we have some "evidence" to examine.
On a practical level, someone could come along in Family Tree and change the birth information for my Samuel Linton born in 1775 without putting in a source and how would I know if what was entered was correct?
There are a much more complicated reasons for adding sources or citations but the all boil down to substantiating the information in a way that others can verify what we have entered. If I go back on my family lines in the FamilySearch.org Family Tree, I will very quickly find entries with no sources. What about your own entries? How many of them have even one source to substantiate what is in the Family Tree?