A Tea Addict's Journal

Logic problem

June 14, 2009 · 8 Comments

Everybody have seen this before

All dogs have four legs
My cat has four legs
Therefore, my cat is a dog

I think we can all spot the problem here — my cat could be a dog, but since not only dogs have four legs, it does not have to be.  In fact, my cat is not a dog because of traits unrelated to the four legs.

Now, let’s try this

All good tea has X
This tea has X
Therefore, this tea is good.


All old teapots has Y
This teapot has Y
Therefore, this teapot is old

Now, these statements can all be true — they are potentially true.  However, as we’ve seen in the first problem, they do not have to be true.  Quite the contrary, in fact.

A real life example that is widely used is this

All old tree puerh leaves have thick veins
This tea has thick veins
Therefore, this is an old tree puerh leaf

At one point I subscribed to this theory, or at least strongly entertained the possibility of it, but upon further reflection and observation, I have found this to be untrue.  I have seen teas that are obviously from large plantations (big factory stuff) that exhibit thick veins, therefore disproving this theory that thick veins prove a tree’s age.

It is pretty easy to fall into the trap of following along one of these flawed deductive reasoning, usually from a reputable seller or vendor or “expert” and then just taking the statement at face value and not thinking through the logical implications of the deductive process.  While it certainly may have been true that only old tree leaves have thick veins, there is no guarantee that this was the case without extensive evidence that all other kinds of tea tree leaves have no thick veins.

Another one

All old teapots are tea-stained
This teapot is tea-stained
Therefore this teapot is old.

Obviously it doesn’t have to be true again.

Now, with other supporting evidence, these statements could be true.  If we assume that zhuni is now extinct and has been for decades, for example (a point that is hotly debated everywhere), then we can probably say

All zhuni pots are made from clay that is extinct
This teapot is a zhuni pot
Therefore this teapot is made from clay that is extinct

Ok, that works, but if you change it to

All zhuni clay is decades old
This teapot is made with zhuni
Therefore this teapot is decades old

Well, somebody may point out that a potter may have harvested a lot of zhuni clay before it went into extinction, and is in fact still producing new pots using this old clay.  So, even though your clay is decades old, the pot is brand new.  In theory, this is possible.  In practice, how anybody can store (securely, I might add) tonnes of clay that seems inexhaustible is questionable.  Either way, the above statements do not convey the entire argument that will have to go into debating whether a pot is new or old.  Using one small trait as its definining characteristic is not exactly reliable if you don’t know all the other relevant facts.

This kind of reasoning works better in the other direction, actually.  Let say somebody devised a new way of pressing puerh cakes that embosses a mark on the cake itself

All embossed cakes are new
This cake is embossed
Therefore this cake is new

That would work since we know that the embossing process is new.  The first line should actually read

All embossed cakes can only be new

Then there’s no doubt as to what’s going on.

The problem with processes doesn’t work the other way though.

All old cakes were stone pressed
This cake is stone pressed
Therefore this cake is old.

Just because people used to do things a certain way doesn’t mean that somebody living now cannot recreate the same process, in this case pressing tea with stone moulds.  In fact, we know this is happening everywhere as tea makers revived the stone-pressed cake since the 1990s.

I guess the point of this post is — beware of these logic deductions based on one or two traits of whatever good that is being sold.  We all know that the job of the vendor is to sell you things.  It’s very easy to fall into the trap (as I did with the thick veins thing) of just assuming this to be true and then not realizing that it, in fact, is not.

Categories: Old Xanga posts

8 responses so far ↓

  • Anonymous // June 14, 2009 at 3:56 pm | Reply

  • lewperin // June 14, 2009 at 6:14 pm | Reply

    Very nice post. A little logic never hurt anyone!

    You aroused a skeptical thought that occurs to me from time to time. The statement

    All old tree puerh leaves have thick veins

    seems to be believed everywhere, and honestly I’ve never seen evidence to the contrary. But is it true, really? Has anyone seen a persuasive botanical explanation for why this would happen?

  • MarshalN // June 14, 2009 at 6:17 pm | Reply

    @lewperin – 

    I don’t believe there is any basis to that claim, and do not subscribe to that theory. I also have never seen any evidence that proves it this way other than anecdotal evidence. In addition, the implication is that old tea trees are of higher quality — which is another step from the claim — is not proven. Basically, what the equation should look like, if you believe it, is

    thick veins = old tea trees = better tea

    Neither of these steps, IMO, are guaranteed to be true and I certainly don’t believe it.

  • Anonymous // June 15, 2009 at 3:48 pm | Reply


    I was in correspondence the other day, and I was set to wondering about the new tree versus old tree idea. I would be happy if you would expound on that a bit. I keep hearing how there are 1000-year-old trees in China (or however old), and the teas from these leaves are much better, because they’re more complex, and so on.

    And then I was reading a Darjeeling grower who was quite happy with his young trees, and saw that as an advantage in his market. He was happy that his trees were all “under 5 years old.” And that’s what made me start to wonder.

    You say there is no proof in the superiority in old tea trees, except anecdotally. But would not anecdotal evidence be the only way we can judge something like the flavor/mouthfeel/Qi of a tea? I’ve never heard of chemical analysis that would be able to do that.

    Thank you for your always informative and interesting blog.


  • MarshalN // June 16, 2009 at 5:40 pm | Reply

    Steven: Good question, and one for which I have no good answer. The same is true for a lot of Taiwanese farmers — they want young trees, not old ones. I think in this case the young trees have better yield, so it’s purely an economical argument.

    As for the issue of whether or not it’s all anecdotal… you’re correct in that they are, by definition, anecdotal. However, what I probably should’ve said was that for the claim of these teas’ aging potential, i.e. old tree outperforms young trees, there is still no proof at all that this is necessarily so. We haven’t really had enough time to test whether or not 30 years old cakes made with old trees are really better than 30 years old cakes made with plantation trees yet. The antique cakes were all made with old tree, some might say, but that was back then. Conditions are very different now, and at any rate, we have no control for those antique cakes, so we really don’t know how plantation tea (as grown in the style currently done) would have fared over that long a period.

  • lewperin // June 16, 2009 at 6:13 pm | Reply

    We haven’t really had enough time to test whether or not 30 years old cakes made with old trees are really better than 30 years old cakes made with plantation trees yet. The antique cakes were all made with old tree, some might say, but that was back then.

    So, 30 years ago, Menghai et al. were buying all their maocha from individuals who climbed big trees near their villages? The plantations are all more recent than that?

  • MarshalN // June 17, 2009 at 12:10 pm | Reply

    No no, when I said antique…. I was thinking of pre-WW2 stuff. Before Menghai et al even existed in their nascent form.

  • mulcahyfeldman // June 17, 2009 at 10:15 pm | Reply

    All dogs have four legs
    My cat has four legs
    Therefore, my cat is a dog

    Actually, my 9-month-old grandson agrees with this concept completely. Best, eillen

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