It seems that logicians are of differing opinions regarding the existential import of propositions. The "modern" view holds that universal propostions (All X is Y) do NOT assert the existence of their subjects but that particular propositions (Some X is Y) do.
I'm confused about this. It seems to me that in neither case can you assume anything; i.e.: sometimes you can and sometimes you can't assume that a subject "exists". In your book "Future Logic" you don't go into this (at least, I haven't been able to find any view one way or the other), so is this essentially a non-issue for you? and if so, why?
Thanks.

This topic is dealt with in Future Logic in Chapter 41, under the heading of Modalities of Subsumption.
http://www.thelogician.net/2_future_logic/2_chapter_41.htm

The existential import theory is rather limited because it only distinguishes between actual being and non-being, whereas in fact the way we use terms in practice is more complex, involving other possibilities.

Of course, an underlying issue is the nature of terms - i.e. concepts.
For that, look at my Phenomenology, chapter 4.
http://www.thelogician.net/2b_phenome_nology/2b_chapter_04.htm

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This essay explains why the existential import doctrine of modern logic is really trivial. It gives equal weight to empty terms, putting them on a par with realistic terms. Whereas in actual logical practice, once a term is classed as empty it ceases to have any significance in scientific and other realistic thinking. Read the essay!