Correction:

Roy Lewallen wrote:
. . .
A passive lossless filter can't achieve any frequency selection by means
of loss, so it depends entirely on mismatch. Among other things, that
means that a passive filter works properly only when both the source and
load impedances are the ones it was designed for. A lossless lowpass
filter has zero loss only at DC. . .

The last sentence should read:

A lossless lowpass filter has zero attenuation only at DC.

The attenuation is often called "mismatch loss", but in the remainder of
what I wrote, I use the term loss only to mean dissipative loss -- which
"mismatch loss" isn't.

Roy Lewallen, W7EL