Lay opinion testimony is tolerated because, frankly, it is hard to find the line between fact and opinion (“she is tall,” “they appeared drunk,” “the car was way over the speed limit”) and because lay witnesses should be comfortable speaking in normal, conversational, easily understood terms. To ensure there is a foundation for the opinion rather than mere speculation, there is the testing device of cross-examination.

But lay witnesses may not cross a line and give testimony that is “based on scientific, technical, or other specialized knowledge within the scope of Rule 702.” See Pa.R.Evid. 701. That’s for experts, and with expert testimony comes discovery obligations such as reports and CVs. The line is ill-defined. And the language of 701 itself is blurry—when it says “specialized knowledge within the scope of Rule 702” does that mean that all specialized knowledge is within 702 or there are types of specialized knowledge, some covered by 702 and others not? The answer is probably the latter, since some recognized forms of lay opinion have to be “specialized.” When a witness says “Jules Epstein wrote that note” their claim is “specialized” to the extent that they, and not the jury, have “specialized” familiarity based on receiving mail from me or watching me write on the board in a lecture hall.