My particular, specific collecting philosophy, and my "universe" of what I collect.
When I started collecting I developed an interest in old stuff that was a challenge to shoot for lack of ammo. In those days nobody was making brass for non-US stuff, and so brass had to be hand-made from other more common calibres (e.g., .348 Winchester, .50-70, .500 Nitro when it could be found). But I kept running into people who had "collections" consisting of a few rifles, a few pistols, some revolvers, a bunch of rifles both military surplus and sporting and other than "that's nice" they made no sense. These were accumulations, not collections. So one day I sat down and said: "What do I want to actually collect?" I decided that I liked the following: 1) being able to shoot. 2) Enjoying metallic cartridges because I like reloading them 3) Enjoying the creativity, engineering genius and variety of problem solving being displayed in the latter 19th Century, 4) collecting US martial arms was just too crowded and expensive!
As I had done coin collecting and stamp collecting in my youth, and had dabbled a bit in cartridge collecting, I came to the strong realization that I didn't want an open-ended collection that had not possibility of an end-point. I wanted something that I could at least get "most" of. In those early days I decided that I wanted one-each of every kind of military action from the black powder cartridge days. I literally sat down and, using John Waters first edition of Rifles of the World, I wrote out myself a list of everything that would constitute such a collection (much like a coin book of Indian Head pennies, or a stamp book of stamps of Canada or something). That list became my "universe." I put my focus on that.
It's true that soon I yearned for more than one bolt action, and damn if I didn't have a couple of different Rolling Blocks (a Spanish AND a Danish, and then a damned Egyptian!! So I re-did my universe, defined it carefully, and in that definition I've been happy: "Non-US issued, black powder metallic cartridge, military rifles." THAT'S my Universe. So I don't collect US issued stuff ... too many and too expensive. I don't collect muzzle-loaders. But the damned paper cartridge needle rifles sometimes tempt me, the Dryese, the Chasspot, the Italian early Carcano needle rifle. Oh well, I decided that you can't collect everything. I only collect long arms, and I don't collect carbines. Although here again it's tough. What about the Fruwirth, the first repeating long arm, but built only as a gendarmerie, a very short rifle, never built as a long rifle? What about the Russian cavalry rifles, which are fairly long, much longer than carbines? Well, I've just had to make judgement calls at the edges, but mostly I've stuck to the collection description and that has given me focus and expertise. It's limited, I don't know everything, but I do know what I know, and know it well.