What is ijtihad?

Adopted from the book : "The Principles of Ijtihad in Islam" by : "Shahid Murtadha Mutahhari"

The question of ijtihad is a very topical one these days.10 Many people ask, either aloud or to themselves, what form ijtihad takes in Islam, and from where Islam got the concept. Why should one practice taqlid? What are the conditions for ijtihad? What are the duties of a mujtahid? Broadly speaking, ijtihad has the meaning of being an authority in the matters of Islam; but there are two ways of being an authority and deriving opinions in the matters of Islam in the eyes of us Shi'i Muslims: one which is in accordance with the shari'a, and one which is forbidden by it. Similarly, taqlid is of two kinds: one which is in accordance with the shari'a, and one which is forbidden.

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10. This address was given on 1 Urdibihisht 1340 Sh. (21 April 1961), three weeks after the death of Ayatollah Burujirdi.