I think I speak for all genealogy researchers when I say that primary sources get me all jazzed-up. I'm over the moon when I get my hands on an original document especially one written in an ancestor's own handwriting. Letter, diaries, family bible entries put me in such a state it would make you blush.
A primary sources is a source of evidence created at the time of the event. For example, a death certificate is a primary source for an individual's date of death, the cause, and the location at which the death took place. A death certificate is NOT a primary source for the individual's date of birth even though it often contains an accurate date of birth. A diary is a primary source for the writer's day-to-day experiences.
A secondary source provides interpretation and analysis of primary sources. Secondary sources are one step removed from the original event. So back to that death certificate, that is a secondary source for the date of someone's birth because the informant provided the most accurate information that they knew which of course is not always accurate. My grandfather always lied about his age; at the time of his death his own children weren't sure what year he was born. A book is a secondary source; the author may use that diary mentioned above as a resources for insight into writing about a historic event but the book produced is a secondary source.
There are even something called tertiary sources which I won't bother your pretty little head with...
As a genealogy researcher, one wants all the primary sources he/she can get; not only for that super tingly feeling one derives but also to be able to state facts. A death certificate let's a researcher say, "He did die on June 1st. His birthday may have been March 2nd. He was probably 90 years old when he died."
Facts are what researchers want. There are many instances though when primary sources just don't exist though. For example, civil registration of births did not begin in New York state until the later half of the 1800s. So your relative that was born in New Amsterdam in 1640 isn't going to have a birth certificate. A birth register may have been kept by the religious institution your family belonged to and perhaps you'll be fortunate to find such a resource still in existence in some archive somewhere. However, a lot of primary sources have been lost to the ravages of time; fires, floods, poor conditions, and handling.
So how do you know?
Well, you may never know. That is just the coldest hard fact. You may never know.
Can secondary sources be trusted?
Genealogy research does, however, require a wee bit of trust. Just because a source is not a primary sources does not mean it is wrong. If that were the case, why would we have any books at all?? Secondary sources CAN be incredibly reliable. Just remember to examine your "facts" relative to the sources.
In my next post I'll provide and example that clarifies exactly what I mean by "examine your 'facts' relative to the sources." Until then.................be skeptical.