I quickly wrote this up to replace what I had before, which I thought was poorly written, and which I don't agree with as much. But you can read it here if you want.
A few days later (30 January 2021)... I wrote up one more try to define fiducialism... I agree with that new one and this one that you are reading, but the new one probably is slightly better conceptually. But this one connects to effective altruism a little better. The 2018 booklet on fiducialism goes into more detail, in a more "Continental philosophy" style, and I can recommend it still. I am struggling to make a definitive version, as you can see, but all my attempts have significant overlap, so you hopefully should be able to have a fairly good idea of what "fiducialism" is.
Receptivity to Reality is an important addendum to the fiducialism posts.
As altruists, we seek human well-being. How should we define "well-being"? There are different inputs into this definition, including the popular hedonism and preferentialism. This post describes another, called "fiducialism".
"Fiducial" in this context means "pertaining to trust". Trust (following Joseph Godfrey's definition in Trust of People, Words, and God) is "receptivity to enhancement". A fiducial utilitarianism would seek to maximize trust. It would try to facilitate or encourage an increase in receptivity to enhancement in humans (or animals, if that can be measured). It would also try to minimize betrayal. "Betrayal", by my definition, is "an insult to the organ of trust", as though the brain (or the body) were organs of trust which could be stressed, shocked, paralyzed, etc. by betrayals. After a betrayal, for a while, or even permanently, someone who has been betrayed can't trust, or trust to as great an extent.
Fiducialism opposes a narrowing of horizons and a settling on what is less than the best. Hedonism and preferentialism are consistent with this narrowing and settling. Humans who adopt hedonism can always quit seeking reality once they've got their wealth (their secure pleasure). Who would really want to seek for the greatest pleasure? Those who do are being fiducialists, in perhaps an immature (child- or adolescent-like) way, if they choose the "smaller infinity" of "infinite" (maximized) pleasure over receptivity to reality. Fiducialism is in favor of the truth in itself. To trust is to be personally open to what is outside yourself, which extends to both facts and people. Fiducialism favors depth and opposes complacency.
Some betrayals are bad, some are good, in the long run, for increasing trust. Sometimes being broken down opens us up, sometimes it doesn't. To trust heroically involves trusting life when it is at its worst. Finding the best in what is bad and rigorously valuing it. Valuing all the good. Pain can be a betrayal, so can confusion. Physical death prevents trusting (to the extent that it is the permanent end of a life), although death can be trusted. Fiducialism avoids pain and death to some extent, like hedonism. A fiducialist should be prone to enjoyment, greater enjoyment than a hedonist to the extent that their attitude of openness to what is makes them more receptive to the pleasure of reality, although there may be more pain in their lives as well.
Ultimately, trusting is a disposition, and not a flow of experience. So fiducialism is person-focused and not experience-focused. Is a person inclined to being receptive to enhancement? Then they are trusting.
Unofficially, people are already hedonists, preferentialists, and fiducialists, (and other "-ists") in some mixture. An altruist is likely to be more fiducialist than hedonist, unofficially, although they might hold to hedonism. Idealism is more fiducialist than it is hedonist or preferentialist.
I would say that if we were trying to pursue altruism without recourse to technology, promoting fiducialism would be a priority. (If we have to work with humans, we would want a more fiducialist population.) I'm not sure that is the case in the future. Technology may make it so that we no longer have to rely on human attitudes and spirits (for better or worse). But whatever we do, we shouldn't narrow our horizons in favor of what is convenient. One might think expanded horizons applies to exploring outer space, but I would like to say that more important is exploring the true nature of inner space, of being a personal being.