Recently, on my first trip to Australia, I finally tasted Vegemite. At the time, I didn't realise I was having a philosophically significant experience, but according to the American academic LA Paul, I was. She uses the example of Vegemite to illustrate something that seems obvious, but that's actually rather intriguing, about "phenomenal knowledge" – the knowledge of what it feels like to experience something. The intriguing point is this: you can obtain such knowledge only from experience. No matter how much information I might be given by others about what Vegemite tastes like, that information can never amount to experiencing the taste itself. By the way, Vegemite tastes a lot like Marmite. I know: major anticlimax.

OK, but who cares? In a paper to be published in the journal Res Philosophica, Paul argues that you should care – because the problem applies to major life decisions, too, where its consequences are graver. As you'll have noticed, every week seems to bring a new magazine story or book making the case for having children or not, marrying early or late and so on. These quote anecdotes and statistics aplenty; the implication is a reader could weigh the evidence, then make a rational decision. But Paul thinks we can't. She calls these big decisions "phenomenologically transformative": experiencing them changes you so much that you become a different person, and prior to the experience, you can't know if that future person would enjoy it. Or, to quote a summary of Paul's paper, co-written with the sociologist Kieran Healy: "Without just the sort of self-knowledge that you get from your own experience of having a child, you can't know how the experience will affect you."

So you can't know for sure. But couldn't you make a good guess, by looking at people otherwise similar to you, and asking how their choices worked out? One of several problems with this is that those people can't know if their choices "worked out", either, and for a similar reason: the child-free person has no experience of parenthood, while the parent has no experience of growing older child-free. (Besides, if parenting makes you a different person, perhaps that includes making judgments about happiness using different criteria.) You could look at the population as a whole instead. But if you're leaning one way, would you really switch to the other based on such a study? (On parenting, many studies continue to defy conventional wisdom, suggesting that parents are less happy.) In reality, nobody makes such decisions with no regard for their own feelings.

There's some debate over whether Paul's argument really proves the technical point, about rationality, that she is seeking to make. But there is no doubt that it's a reminder of how much we're in the dark when it comes to life's most important decisions. If an experience will make you a different person, you can't know in advance what that person will make of it. This arguably applies even to tiny changes – taking a different route to work, say. For bigger ones, it suggests that "rational decision-making" may be nothing but a comforting fantasy; all we're really doing is taking blind leaps of faith. Paul and Healy quote a profound observation, attributed to the philosopher-comedian Steven Wright: "Experience is something you don't get until just after you need it."

oliver.burkeman@theguardian.com

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