The Doctors of the Church like to reference each other, especially when Saint Alphonsus is involved. Even when someone was not a Doctor of the Church yet when he was alive, this didn’t seem to stop him from being particularly interested in what they said. He has a knack for that.

In an appendix to Selva, we get the following scheme from Alphonsus Liguori:

Cardinal Bellarmine thinks it to be morally impossible for any Christian to fulfil the duties of a simple Christian without mental prayer [meditation and/or contemplation]… My dear patroness St. Teresa also says that to obtain God’s grace the only door is mental prayer, and she speaks expressly of mental prayer, “This door being closed,” she says, “I know not how grace will reach the soul.” (Life, ch. 8)

Saint Robert Bellarmine, Saint Teresa, and Saint Alphonsus in one short passage: “Mental prayer is prescribed to all Christians. We’re not sure how you’re going to get through without it. Go for it!”