Learn English – “The (Cobra)” vs. “An (elephant)”, articles with nouns denoting a class

definite-articlesindefinite-articlesuncountable-nouns

[ 1 ] tells on p.5 that "Singular nouns denoting a class" are preceded by the definite article "THE" (Example: "The Cobra is dangerous"), while on page 7 (Table 6. THE INDEFINITE ARTICLE) it tells that "Singular countable nouns denoting a class" are preceded by indefinite articles ("An elephant never forgets").

I could not get what is the difference between the two cases mentioned.
Why can't I write according to the same rules "A Cobra never forgets" and "The elephant is dangerous"?

[ 1 ]
PRACTICAL REALISATIONS OF ENGLISH DEFINITE AND INDEFINITE ARTICLES

p.5 (of 8)

=================================  
 4. THE DEFINITE ARTICLE  
----------------------------------   
 USES          EXAMPLES 
-----------------------------------
 Singular        The Cobra is dangerous    
 nouns 
 denoting 
 a class
=======================================

p.7 (of 8)

=================================  
 6. THE INDEFINITE ARTICLE  
----------------------------------   
 USES          EXAMPLES 
-----------------------------------
 Singular        An elephant never forgets countable     
 countable 
 nouns 
 denoting 
 a class
=======================================

Update:
Irene asks:

How about the following example:

Is the happiness of the
majority more important than the
rights of the individual?

According to "First Certificate
Language Practice" by Michael Vince
this is the only way to say it. But
couldn't you also say:

Is the happiness of a majority
more important than the rights of an
individual?

As I see it majority and individual
both represent a class and an example
of a class.

Best Answer

You can use either. There is functionally no difference between the following sentences:

The elephant will flee when confronted by danger.

An elephant will flee when confronted by danger.

Moreover, you can also use the plural with no article to say the same thing:

Elephants will flee when confronted by danger.

(Sorry, my phone doesn't want to let me select blocks of text for some reason, so I'm unable to highlight these examples.)

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