Actuosa partipatio

Pope St. Pius X has written:

The Holy Mass is a prayer itself, even the highest prayer that exists. It is the Sacrifice, dedicated by our Redeemer at the Cross, and repeated every day on the Altar. If you wish to hear Mass as it should be heard, you must follow with eye, heart, and mouth all that happens at the Altar. Further, you must pray with the Priest the holy words said by him. You have to associate your heart with the holy feelings which are contained in these words and in this manner you ought to follow all that happens on the Altar. When acting in this way, you have prayed Holy Mass.