use of definite article

[Carvalho]

I have a question, should we use definite article "the" in this sentence:

On a trip we made to London, we visited St Paul's Cathedral.
or:
On a trip we made to London, we visited the St Paul's Cathedral.

Can you explain me the use or not of "the" in this case? Thanks!!

[Cerberus™]

Whether or not to use the definite article with unique buildings is, as far as I know, fixed idiom without any clear rules. But in this case the established idiom "Saint Paul's Cathedral" (no article) can be defended: the cathedral is already defined by the possessive. Compare "I like Napoleon's horse": it is not "/the Napoleon's horse/", because the question "which horse?" is already answered by "Napoleon's!" and hence doesn't need an article.

[Harry_Ph]

I absolutely agree! But the definite article is needed if it is stated as "Saint Paul Cathedral," i.e., the Saint Paul Cathedral.
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The more we learn, the less we seem to know.

[Cerberus™]

Well, it is always called "Saint Paul's Cathedral"; the article is perhaps used with other cathedrals.

[Harry_Ph]

My example “Saint Paul Cathedral” was not intended to refer to that particular cathedral in the City of London. Sorry, Cerberus, if my point was not clarified.
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The more we learn, the less we seem to know.

[Cerberus™]

OK clear.