What is the difference between doctrine and discipline?

I have had the pleasure of sharing my faith with a non-Catholic friend of mine. And she posed the question: What are the guidelines for determining what is doctrine and what is discipline?

As a rule of thumb, doctrine is what we believe and discipline is how we live out what we believe. For example, that women cannot be priests is doctrine because it is an expression of the nature of the Christian priesthood; that priests of the Latin rite of the Church must be celibate is a discipline because the Church has determined that celibacy is a helpful rule of life for priests. Because doctrine is an explication of the truths of the faith, it cannot change; because discipline regulates Christian spirituality and worship, it can.

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