May 04, 2019 Saturday
Bedtime Story
Cause: Necessary and Sufficient
Tonight we shall study causality in its
simplest logical form.
Causes can be broadly seen to be either
necessary ones or sufficient ones.
What they mean exactly would be rather well
understood from the examples rather than their definitions.
If a factor x is a necessary cause of an
event y, then the presence of y necessary implies that x preceded it.
But still that does not mean that if the
factor x were to be present the event y will occur.
And yet for y to have occurred it is
necessary that the factor x preceded it.
For instance for a person to develop full
blown AIDS with vision threatening conditions such as cytomegalovirus retinitis
or necrotizing herpetic retinitis due to varicella zoster virus it is necessary
that the person was infected with HIV.
(Varicella-zoster virus is a neurotrophic
virus preferring the ganglia of the central nervous system to inhabit of which
retina is also a part but because of the close association of microvessels to
nerves it affects the vessels too leading to their occlusion and thereby
leading to tissue ischemic necrosis.)
However, we also are fully aware that not
all patients who are infected with the HIV virus will develop full blown AIDS
for various reasons.
This means that HIV infection is a
necessary cause of full blown AIDS but not a sufficient one needing other
causalities to come into play.
In contrast to the necessary cause the
causality of sufficient cause works in a slightly different way.
In this case when factor x is a sufficient
cause of event y to occur then the presence of y does not necessarily imply
that x happened before y as there could be another factor such as a or b that
could have caused y.
In this case y would definitely follow x
but not the other way around.
This means that if the factor x exists that
it is certain that event y will occur.
A simple example is that severe and
prolonged obstruction of trachea (as in hanging) will definitely lead to death
of an organism.
But this does not imply that all deaths are
due to obstruction of trachea.
Therefore prolonged obstruction of trachea
is a sufficient cause of death but not a necessary one.
There can then be other combinations of
these two types of causes such as neither necessary nor sufficient and both
necessary and sufficient.
I will not load you with examples for these
combinations and in fact I would encourage you to think real-world examples up
for yourself.
Thus all events that we see around us must
have either a necessary cause or a sufficient cause or their combinations.
Stay tuned to the voice of an
average story storytelling chimpanzee or login at http://panarrans.blogspot.com
Good night Mon Ami and my fellow cousin ape.
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Another great educator and a teacher that I am aware of is
Professor Subhashish Chattopadhyay in Bangalore, India.
While I narrate stories, Professor Subhashish an electronic
engineer and a former professor at BARC, does and teaches real mathematics and
physics.
He started the participation of Indian students at the
International Physics Olympiad.
Do visit him here:
All his books can be downloaded for free through this link:
For edutainment and English education of your children, I
recommend this large collection of Halloween Songs for Kids:
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