This paper; https://arxiv.org/abs/1701.07261, Einstein's 1917 Static Model of the Universe: A Centennial Review, offers an informative insight into the mind of Einstein and, to large extent, his motivation for adding the cosmological constant to his field equations. Interestingly, both the introduction of lambda, and his subsequent repudiation, inspired controversy among from his peers. Though the basic idea had been around for decades, it had never been expressed quite so eloquently. The paper goes on to address the issues he was attempting to resolve with lambda. The jury is still out on whether or not it was justified. It is clear, however, Einstein was deeply ambivalent towards lambda for the remainder of his life. While lambda may have enabled Einstein to predict the expansion of the universe, it mainly served to resurrect an issue well known and argued since the time of Newton - that a static universe was unstable against gravitational collapse.