W,
It seems to me that how "far back" you search and how broadly you search are two different questions that have more or less the same answer:
Anything considered prior art according to 35 USC § 102 or 103 is fair game.
MPEP § 706.2 gives a good summary of the difference.
"DISTINCTION BETWEEN 35 U.S.C. 102 AND 103
The distinction between rejections based on 35 U.S.C. 102 and those based on 35 U.S.C. 103 should be kept in mind. Under the former, the claim is anticipated by the reference. No question of obviousness is present. In other words, for anticipation under 35 U.S.C. 102, the reference must teach every aspect of the claimed invention either explicitly or impliedly. Any feature not directly taught must be inherently present. "
Using your toothbruse example, this says essentially that if a similar toothbrush was known to the public prior to your invention, you can't get a patent.
This might be thought of as the answer of how "far back" to look.
The MPEP goes on:
"Whereas, in a rejection based on 35 U.S.C. 103, the reference teachings must somehow be modified in order to meet the claims. The modification must be one which would have been obvious to one of ordinary skill in the art at the time the invention was made. See MPEP Section 2131 - Section 2146 for guidance on patentability determinations under 35 U.S.C. 102 and 103."
http://www.bitlaw.com/source/mpep/706_02.htmlUsing your same example, this says in effect that any brush (or combination of brushes) known to the public prior to your date of invention is prior art.
This might be thought of as the answer to "how broad" you should search.
In practice, I think there is a pretty wide lattitude about how these are interpreted. I've seen both US examiners and EPO search departments come up with some pretty creative stuff under both of these categories.
Regards,
eric stasik