# Science like a chef

Alice: Hey honey, I made pasta with tomato sauce!

Bob: Great, let's eat!

Bob: Mmmmm, that's fantastic! It's even better than last time. It's got a sweeter, deeper flavor, which I like.

Alice: Thanks. Last time I only sautéed onions and garlic before adding the tomato puree, but this time I added carrots to the mix for some extra sweetness.

Bob: You know I love your cooking, but I always feel a twinge of skepticism whenever you try to explain why things taste the way they do. Yes, you added carrots to the sauté. But is that the only thing you did differently?

Alice: No. I added some butter along with the olive oil to give it some smoothness. Ummm. I threw some red wine in to the sauce. We didn't have any basil leaves today so I just left that out of the recipe.

Bob: So how do you know that the _carrots_ are what made it taste sweeter? How do you know that it wasn't the butter, the red wine, or the absence of basil leaves?

Alice: Well, the red wine is acidic, so that wouldn't make it taste sweet. I think the butter helped make it taste richer and smoother. The basil leaves _add_ flavor, so leaving them out probably subtracted from the deepness, rather than adding to it.

Bob: Sure, that all sounds plausible. But how can you _know_ any of this?

Alice: It's just common sense. I know that butter is rich. I know that red wine is acidic. I know that basil is flavorful. So I know that adding them will have the corresponding effects on the sauce.

Bob: But isn't it possible that things work differently when you _combine_ them? Sure, red wine is acidic by itself, but isn't it possible that when combined with sautéed onions, garlic and carrots, and olive oil and butter, and fried tomato paste, and whatever spices you used, and cooked. Isn't it possible that after all of that, some chemical reactions occur that cause the red wine to be sweet instead of acidic?

Alice: I don't think so. I think the sweetness came from the carrots. But yeah, I suppose it is _possible_. So what do you propose I do?

Bob: Cook like a scientist. Only change one variable at a time. Take note of the effect. So in this case, if you hypothesize that adding carrots to the sauté will add sweetness to the sauce, _just_ add carrots to the sauté. _Don't_ add red wine, or anything else.

Alice: Ok, let's suppose I did do that, and I found that carrots did in fact add sweetness. What if I want to add red wine next time. Isn't it possible that carrots add sweetness to the basic sauce, but when combined with red wine, carrots react with the red wine and actually add, I don't know, bitterness to the sauce?

Bob: Yes! You're cooking like a scientist now!

Alice: So I'm supposed to then do _another_ experiment? Make the sauce with red wine, make the sauce with red wine and carrots, and see what the effect of carrots are in that particular scenario?

Bob: Absolutely.

Alice: And then what if I want to use ghee for my fat instead of olive oil?

Bob: More experiments.

Alice: Don't you think that so much experimenting is impractical?

Bob: Maybe. But it's the only way to know the truth. The laws of scientific inference don't care how easy they are to follow. They're _laws_.

Alice: Maybe. Have you ever heard of bayesian inference?

Bob: Uh yeah, I think so. I think they had a brief section on it in one of my undergrad stats classes. I remember something about it helping solve a game show problem. But we don't use it much in our work at the lab.

Alice: Well the reason I bring it up is because you seem to view inference as a binary thing. Either you conclude that carrots added sweetness to the sauce, or you don't. That's not how I, or other bayesians see things. To me, everything is a spectrum. Of course I can't say that I'm _100%_ sure that the carrots were the cause of the resulting sweeter flavor, but I will say that I'm about 90% sure. Yes, the fact that I added red wine, butter and removed basil introduce the possibility of confounding, but I feel like I have a pretty good handle on these ingredients given my experience cooking, and my judgement is that it's unlikely that they were the cause of the extra sweetness.

Bob: _Sigh_. I mean, I guess you're right. But that's just a very unscientific way of doing things.

Alice: And why is that a bad thing?

Bob: Because with that approach, you can never know for _certain_ what the truth is. Like you said, you're only 90% sure that the carrots caused the sweetness, not 100% sure.

Alice: Well I'm ok with that. In a perfect world, I'd do things your way and only manipulate one variable at a time, but that's just not practical in the kitchen. Like you said, every time I change one thing, I'd have to redo the experiment with the carrots. Given how many variables there are, I'd have to do hundreds, maybe even thousands of experiments.

Bob: Again, proper experimentation is the only way to actually know things with certainty.

Alice: Yes, proper experimentation would allow me to boost my confidence from 90% to 100%[*](http://lesswrong.com/lw/mp/0_and_1_are_not_probabilities/). But the cost of this boost would be hundreds of experiments. Thousands of hours of my time. I don't care enough to do that. The return on investment is totally not worth it. When the return on investment isn't worth it, sometimes it makes sense to do science like a chef.

Bob: Well when I cook, I like to do science like a scientist.

Alice: And _that's_ why your tomato sauce stinks.