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How Does an Exception Prove a Rule?

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Here are a few things that have recently been called “the exception that proves the rule”: the Mini-Transat sailing race  (because it’s international and doesn’t “boil down to duels between French sailors”), Adrian Peterson (because his success as a football player is not about “opportunity and scheme” but unusual talent), and a congressional compromise on student loans (because a congress reached a compromise!). Taken as a set phrase, “the exception that proves the rule” indicates a deviation from the norm, a challenge to the stereotype. It says, in effect, the norm or stereotype is the rule and here is something that is an exception to that rule. But wait, how does the exception prove the rule? Wouldn’t it do just the opposite? Doesn’t it prove that the rule does not hold for all cases and is therefore not a rule at all?

It is sometimes argued that the confusion over this expression stems from the wrong understanding of “prove,” that “prove” here means “test,” as in “proving ground” or a printer’s proof. The idea is that the exception tests the validity of the rule, and that test could either leave the rule intact (if some kind of explanation can be found) or overturn it. However, it’s hard to come up with an example where that is truly what is intended by the phrase. It almost always carries the assumption that the rule remains intact.

In fact, the “prove” part of the phrase was not very important in its original formulation. The expression comes from the Latin legal principle exceptio probat regulam (the exception proves the rule), also rendered as exceptio firmat regulam (the exception establishes the rule) and exceptio confirmat regulam (the exception confirms the rule). The principle provides legal cover for inferences such as the following: if I see a sign reading “no swimming allowed after 10 pm,” I can assume swimming is allowed before that time; if an appliance store says “pre-paid delivery required for refrigerators,” I can assume they do not require pre-paid delivery for other items. The exception here is not a thing but an act of excepting. The act of stipulating a condition for when something is disallowed (or required), proves that when the stipulated conditions do not hold, it is allowed (or not required). The general rules are that swimming is allowed before 10pm and that pre-paid delivery is not required. The fact that exceptions to those rules have been stated confirms those rules hold in all other cases. The full statement of the principle reads exceptio probat regulam, in casibus non exceptis. The exception proves the rule in cases not excepted.

These days, you’d have to be one heck of a stickler to insist that “the exception that proves the rule” only be used in its original Latin sense. It has probably come to its current sense through some blending with the expression, “every rule has an exception.” Most of the time, when people say something is “the exception that proves the rule,” they could just as well say it’s “an exception to the rule.” I would argue, however, that “the exception that proves the rule” does more by highlighting the unusualness of the exception. The Mini-Transat, or Adrian Peterson, or a Congress reaching an agreement are not just outside the norm, they are so far outside the norm they force you to notice what the norm is, or that there is one at all. There is an important kernel of the original sense here. The existence of the exception gives force to the rule. The no swimming after 10 pm sign makes it clear that it’s okay to swim before that. The congressional compromise on student loans makes it all the more clear that congress can’t compromise on anything else. Sure, there’s usually a good bit of hyperbole going on when someone pulls out this expression, but that doesn’t mean it doesn’t make any sense.

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Big Questions
Do Bacteria Have Bacteria?
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Drew Smith:

Do bacteria have bacteria? Yes.

We know that bacteria range in size from 0.2 micrometers to nearly one millimeter. That’s more than a thousand-fold difference, easily enough to accommodate a small bacterium inside a larger one.

Nothing forbids bacteria from invading other bacteria, and in biology, that which is not forbidden is inevitable.

We have at least one example: Like many mealybugs, Planococcus citri has a bacterial endosymbiont, in this case the β-proteobacterium Tremblaya princeps. And this endosymbiont in turn has the γ-proteobacterium Moranella endobia living inside it. See for yourself:

Fluorescent In-Situ Hybridization confirming that intrabacterial symbionts reside inside Tremblaya cells in (A) M. hirsutus and (B) P. marginatus mealybugs. Tremblaya cells are in green, and γ-proteobacterial symbionts are in red. (Scale bar: 10 μm.)
Fluorescent In-Situ Hybridization confirming that intrabacterial symbionts reside inside Tremblaya cells in (A) M. hirsutus and (B) P. marginatus mealybugs. Tremblaya cells are in green, and γ-proteobacterial symbionts are in red. (Scale bar: 10 μm.)

I don’t know of examples of free-living bacteria hosting other bacteria within them, but that reflects either my ignorance or the likelihood that we haven’t looked hard enough for them. I’m sure they are out there.

Most (not all) scientists studying the origin of eukaryotic cells believe that they are descended from Archaea.

All scientists accept that the mitochondria which live inside eukaryotic cells are descendants of invasive alpha-proteobacteria. What’s not clear is whether archeal cells became eukaryotic in nature—that is, acquired internal membranes and transport systems—before or after acquiring mitochondria. The two scenarios can be sketched out like this:


The two hypotheses on the origin of eukaryotes:

(A) Archaezoan hypothesis.

(B) Symbiotic hypothesis.

The shapes within the eukaryotic cell denote the nucleus, the endomembrane system, and the cytoskeleton. The irregular gray shape denotes a putative wall-less archaeon that could have been the host of the alpha-proteobacterial endosymbiont, whereas the oblong red shape denotes a typical archaeon with a cell wall. A: archaea; B: bacteria; E: eukaryote; LUCA: last universal common ancestor of cellular life forms; LECA: last eukaryotic common ancestor; E-arch: putative archaezoan (primitive amitochondrial eukaryote); E-mit: primitive mitochondrial eukaryote; alpha:alpha-proteobacterium, ancestor of the mitochondrion.

The Archaezoan hypothesis has been given a bit of a boost by the discovery of Lokiarcheota. This complex Archaean has genes for phagocytosis, intracellular membrane formation and intracellular transport and signaling—hallmark activities of eukaryotic cells. The Lokiarcheotan genes are clearly related to eukaryotic genes, indicating a common origin.

Bacteria-within-bacteria is not only not a crazy idea, it probably accounts for the origin of Eucarya, and thus our own species.

We don’t know how common this arrangement is—we mostly study bacteria these days by sequencing their DNA. This is great for detecting uncultivatable species (which are 99 percent of them), but doesn’t tell us whether they are free-living or are some kind of symbiont. For that, someone would have to spend a lot of time prepping environmental samples for close examination by microscopic methods, a tedious project indeed. But one well worth doing, as it may shed more light on the history of life—which is often a history of conflict turned to cooperation. That’s a story which never gets old or stale.

This post originally appeared on Quora. Click here to view.

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Why Do Cats 'Blep'?
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As pet owners are well aware, cats are inscrutable creatures. They hiss at bare walls. They invite petting and then answer with scratching ingratitude. Their eyes are wandering globes of murky motivations.

Sometimes, you may catch your cat staring off into the abyss with his or her tongue lolling out of their mouth. This cartoonish expression, which is atypical of a cat’s normally regal air, has been identified as a “blep” by internet cat photo connoisseurs. An example:

Cunning as they are, cats probably don’t have the self-awareness to realize how charming this is. So why do cats really blep?

In a piece for Inverse, cat consultant Amy Shojai expressed the belief that a blep could be associated with the Flehmen response, which describes the act of a cat “smelling” their environment with their tongue. As a cat pants with his or her mouth open, pheromones are collected and passed along to the vomeronasal organ on the roof of their mouth. This typically happens when cats want to learn more about other cats or intriguing scents, like your dirty socks.

While the Flehmen response might precede a blep, it is not precisely a blep. That involves the cat’s mouth being closed while the tongue hangs out listlessly.

Ingrid Johnson, a certified cat behavior consultant through the International Association of Animal Behavior Consultants and the owner of Fundamentally Feline, tells Mental Floss that cat bleps may have several other plausible explanations. “It’s likely they don’t feel it or even realize they’re doing it,” she says. “One reason for that might be that they’re on medication that causes relaxation. Something for anxiety or stress or a muscle relaxer would do it.”

A photo of a cat sticking its tongue out
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If the cat isn’t sedated and unfurling their tongue because they’re high, then it’s possible that an anatomic cause is behind a blep: Johnson says she’s seen several cats display their tongues after having teeth extracted for health reasons. “Canine teeth help keep the tongue in place, so this would be a more common behavior for cats missing teeth, particularly on the bottom.”

A blep might even be breed-specific. Persians, which have been bred to have flat faces, might dangle their tongues because they lack the real estate to store it. “I see it a lot with Persians because there’s just no room to tuck it back in,” Johnson says. A cat may also simply have a Gene Simmons-sized tongue that gets caught on their incisors during a grooming session, leading to repeated bleps.

Whatever the origin, bleps are generally no cause for concern unless they’re doing it on a regular basis. That could be sign of an oral problem with their gums or teeth, prompting an evaluation by a veterinarian. Otherwise, a blep can either be admired—or retracted with a gentle prod of the tongue (provided your cat puts up with that kind of nonsense). “They might put up with touching their tongue, or they may bite or swipe at you,” Johnson says. “It depends on the temperament of the cat.” Considering the possible wrath involved, it may be best to let them blep in peace.

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