For over seventy years scholars have accepted the reading advanced by T.-A. Audet, R.M. Grant, and W.R. Schoedel that Irenaeus of Lyons was strictly opposed to causal speculation. This article challenges their reading and offers one of its own. I contend that Irenaeus argues for a differentiated knowledge of terrestrial and celestial matters, such that some things are known while others are not. This differentiation of knowledge encompasses questions of causation. Irenaeus, I argue, was not just concerned about the perils of theological speculation but also its possibilities. For over seventy years scholars have accepted the reading advanced by T.-A. Audet, R.M. Grant, and W.R. Schoedel that Irenaeus of Lyons was strictly opposed to causal speculation. This article challenges their reading and offers one of its own. I contend that Irenaeus argues for a differentiated knowledge of terrestrial and celestial matters, such that some things are known while others are not. This differentiation of knowledge encompasses questions of causation. Irenaeus, I argue, was not just concerned about the perils of theological speculation but also its possibilities.