Tag Archives: logic

Alien Encounter: test your logic skills

Alien Encounter.

The starship Indefensible was in orbit around the planet Noncompostments, and Captain Quirk and Mr. Crock had beamed down to the surface.

“According to the Good Galaxy Guide, there are two species of intelligent aliens on this planet,” said Quirk.

“Correct, Captain —- Veracitous and Gibberish. They all speak Galaxic, and they can be distinguished by how they answer questions. The Veracitors reply truthfully, and the Gibberish always lie.’

‘But physically —‘

‘— they are indistinguishable, Captain.’

Quirk heard a sound, and turned to find three aliens creeping up on them. They looked identical.

‘Welcome to Noncomposimentis,’ said one of the aliens.

‘I thank you. My name is Quirk. Now, you are…’ Quirk paused. ‘No point in asking their names,’ he muttered. ‘For all we know, they will be wrong.’

‘That is logical, Captain,’ said Crock.

‘Because we are poor speakers of Galaxic,’ Quirk improvised, ‘I hope you will not mind if I call you Alfy, Betty and Gemma. As he spoke, he pointed to each of them in turn. Then, he turned to Crock and whispered, ‘Not that we know what sex they are, either.’

‘They are all hermandrofemigynes,’ said Crock.

‘Whatever. Now, Alfy: to which species does Betty belong?’


‘Ah, Betty: do Alfy and Gemma belong to different species?’


‘Right…Talkative lot, aren’t they? Um…Gemma: to which species does Betty belong?’


Quirk nodded knowledgeably.’Right, that’s settled it, then!’

‘Settled what, Captain?’

‘Which species each belongs to.’

‘I see. And those species are —?’

‘Haven’t the foggiest idea, Crock. You’re the one who’s supposed to be logical!’


So, dear students, have some fun!!

More later,

Nalin Pithwa



A mathematician, a physicist and a layman

What exactly is the difference between a mathematician, a physicist and a layman? Let us suppose that they all start measuring the angles of hundreds of triangles of various shapes, find the sum in each case and keep a record. Suppose the layman finds that with one or two exceptions, the sum in each case comes out to be 180 degrees. He will ignore  the exceptions and say “the sum of  the three angles in a triangle is 180 degrees.” A physicist will be more cautious in dealing with exceptional cases. He will examine them more carefully. if he finds that the sum in them is somewhere between 179 degrees to 181 degrees, say, then he will attribute the deviation to experimental errors. He will then state a law, “The sum of the three angles of any triangle is 180 degrees.” He will then watch happily as the rest of the world puts his law to test and finds that it holds good in thousands of different cases, until somebody comes up  with the a triangle in which the law fails miserably. The physicist now has to withdraw his law altogether or else to replace it by some other law which holds good in all the cases tried. Even this new law may have to be modified at a later date. And, this will continue without end.

A mathematician will be the fussiest of all. If there is even a single exception, he will refrain from saying anything Even when millions  of triangles are tried without a single exception, he will not state it as a theorem that the sum of the three angles in any triangle is 180 degrees. The reason is that there are  infinitely many different types of triangles. To generalize from a million to infinity is as baseless to a mathematician as to generalize one to  a million. He will at the most make a conjecture and say that there is a strong evidence suggesting that the conjecture is true. But, that is NOT the same thing as proving a theorem. The only proof acceptable to a mathematician is the one which follows from earlier theorems by sheer logical implications. For example, such a proof follows easily from the theorem that an external angle of a triangle is the sum of the other two internal angles (or, by a suitable construction and using the properties of parallel lines, which in turn can be proved from axioms of plane geometry).

The approach taken by the layman or the physicist is known as inductive approach, whereas the mathematician’s approach is called the deductive approach. 

In the former, we make a few observations and generalize. In the latter, we deduce from something which is already proven. Of course, a question can be raised as to on what basis this supporting theorem is proved. The answer will be some other theorem. But, then the same question can be asked about the other theorems Eventually, a stage is reached where a certain statement cannot be proved from any other proved statements and must, therefore, be taken for granted as true. Such a statement is known as an axiom or a postulate. Each branch of mathematics has its own postulates or axioms. For example, one of the axioms of plane geometry is that through two distinct points, there passes exactly one line. The whole beautiful structure of  plane geometry is based on five or six axioms such as this one. Every theorem in geometry can be ultimately deduced from these axioms.

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Nalin Pithwa