But it is not impossible for a person to distinguish the difference between 80 PPI and 240 PPI (viewed at 6.5 feet). Moreover in most circumstances people won't be restrained at 6.5 feet, and will commonly approach closer if they like the picture.

Yes there are images that can be printed at low resolution, but that is almost always because they are low resolution images to start with, and print just as they are.

It isn't that low resolution can never be used, but the point still should be made very clear that quality images look better at higher PPI values, and planning on never being able to use high enough resolution is a guarantee of lower quality prints.

Your suggestion of resampling high pixel count images to a size that will reduce the PPI to lower than the printers native PPI is a technically very poor procedure. Resampling to a lower value is a very sharp low pass filter that removes high frequency detail (such as sharpening). And it is then always necessarily resampled back up to the native PPI rate for the printer anyway.

HP and Canon print at 300 PPI and Epson at 360 PPI, for example. Resampling a 36 MP image down to 12 MP and then printing it at any size from 11x14 on up is counter productive.

Whatever, Floyd - your logic and interpretations never fail to defy common sense and practice, and your presentation, while sounding impressive, is double talk and obfuscation, mostly. You do make some good points from time to time, but when it comes to theory vs practice - your theory doesn't always result in better prints or images. You are free to enjoy your 300 or 360 ppi prints to your heart's content, with the knowledge that they will look great under that loupe you carry in your shirt pocket. :)