In is a preposition:  and shows the relation between indulge and which; according to Rule 23d, which says, “Prepositions show the relations of words, and of the things or thoughts expressed by them.”  Because the meaning is—­indulge in which—­or, which they indulge in.

Which is a relative pronoun, representing sin or folly, in the third person, singular number, and neuter gender; according to Rule 13th, which says, “When a pronoun has two or more antecedents connected by or or nor, it must agree with them singly, and not as if taken together:”  and is in the objective case, being governed by in; according to Rule 7th, which says, “A noun or a pronoun made the object of a preposition, is governed by it in the objective case.”  Because the meaning is—­in which;—­i. e., in which sin or folly.

The is the definite article:  and relates to multitude; according to Rule 1st, which says, “Articles relate to the nouns which they limit.”  Because the meaning is—­the multitude.

Multitude is a common noun, collective, of the third person, conveying the idea of plurality, masculine gender, and nominative case:  and is the subject of indulge; according to Rule 2d, which says, “A noun or a pronoun which is the subject of a finite verb, must be in the nominative case.”  Because the meaning is—­multitude indulge.

Thoughtlessly is an adverb of manner:  and relates to indulge; according to Rule 21st, which says, “Adverbs relate to verbs, participles, adjectives, or other adverbs.”  Because the meaning is—­thoughtlessly indulge.

Indulge is a regular active-transitive verb, from indulge, indulged, indulging, indulged; found in the indicative mood, present tense, third person, and plural number:  and agrees with its nominative multitude; according to Rule 15th, which says, “When the nominative is a collective noun conveying the idea of plurality, the verb must agree with it in the plural number.”  Because the meaning is—­multitude indulge.

Themselves is a compound personal pronoun, representing multitude, in the third person, plural number, and masculine gender; according to Rule 11th, which says, “When the antecedent is a collective noun conveying the idea of plurality, the pronoun must agree with it in the plural number:”  and is in the objective case, being governed by indulge; according to Rule 5th, which says, “A noun or a pronoun made the object of an active-transitive verb or participle, is governed by it in the objective case.”  Because the meaning is—­indulge themselves;—­i. e., the individuals of the multitude indulge themselves.

But is a disjunctive conjunction:  and connects what precedes and what follows; according to Rule 22d, which says, “Conjunctions connect words, sentences, or parts of sentences.”  Because the meaning is—­A young man, &c., but, ah! &c.