Here’s my summary:
Basically, probabilities are calculated based on the things you know (or think), and obviously not the things you don’t know. For example, if there was a six-sided dice in a box, and someone asked me what I think the probability of it is to have the 1 side facing up…well, the only information I have available is how many sides are on a dice. I don’t have the information about what side actually IS facing up. If I did, I would be able to say “100%” or “0%”. But I don’t have that information. All I know is that the 1 side is 1 out of the 6 available sides. So I can say that there’s a 1/6 chance of the dice being on 1.
Now, that probability must be understood to not exist in reality. The dice either is on 1, or it isn’t. The actual state of the universe isn’t “it’s probably not on 1,” probability only exists in minds, not in reality. The dice either is on 1, or it isn’t.
So, in this sense, probability is subjective. Hence the first part of the phrase “Subjectively Objective.” So how about the latter part? What about it is objective?
Each person has their own “priors” – their own experiences, their own assumptions, their own knowledge, their own beliefs, etc. These things are themselves “subjective” so to speak - they differ from person to person - but it can be said that it is objectively true that your priors are your priors. In other words, if you like vanilla, it’s your subjective opinion that vanilla is good, but it’s objectively true that it’s your subjective opinion that vanilla is good.
So, priors can themselves be subjective, but what your priors are can be said to be objectively your priors. Now, based on what all of your priors are, there is a single correct way to calculate probabilities. In a sense, an objectively correct way to calculate probabilities. For example, in the dice example, one of the priors of mine is that I know it’s a six-sided dice. It would be incorrect for me to take that prior and say “There’s a 1 in 1000 chance of 1 facing up.” The correct answer, based on my priors, is 1/6.
But of course the fact is that people have different priors. Maybe another person is asked the same question – which side is facing up – but this person has additional priors that I don’t have. For example, maybe he’s been asked this question by the same person 50 times already, and so far every time it’s been 1 facing up. Based on this, as well as other priors, he could be all means correctly say that the probability of it being 1 is 90% (I don’t really want to do any real calculations, I wouldn’t even know how to).
Now, does this make my 1/6 calculation incorrect? No! Different probabilities can be said to be correct based on different priors. Priors are subjective. Each person has their own. Some people have “better” priors than others, which will allow them to make better predictions, but that doesn’t mean other peoples’ calculations are incorrect. They’re correct based on the information available, they just had less information available.
So, hopefully I didn’t slaughter the actual message being relayed in the essay. I hope I didn’t misrepresent it too bad. Don’t take my word that that’s what it says. I could have totally misunderstood it. Have fun, and I hope I helped somebody learn something today.