But first, in order to construct the most appropriate Introduction, you must consider the underlying purpose of your speech.
There are four possible purposes: honourable, discreditable, doubtful, and petty. An honourable purpose is when we either defend what seems to be a universal truth, or attack what seems to be universally accepted as reprehensible; for example, when we defend a heroic person, or denounce a heartless killer.
Conversely, a speech might be of the discreditable kind when something honourable is under attack or when something unacceptable is being defended. A doubtful (or ambiguous) purpose is when the purpose of the speech is partly honourable and partly discreditable. Another type of speech is the petty kind – when the matter being discussed is trivial or unimportant.