Samuel Clarke in the context of Book of Common Prayer (1662)

The Kalam cosmological argument is a modern formulation of the cosmological argument for the existence of God. It is named after the Kalam (medieval Islamic scholasticism) from which many of its key ideas originated. Philosopher and theologian William Lane Craig was principally responsible for revitalising these ideas for modern academic discourse through his book The Kalām Cosmological Argument (1979), as well as other publications.

The argument's central thesis is the metaphysical impossibility of a temporally past-infinite universe and of actual infinities existing in the real world, traced by Craig to 11th-century Persian Muslim scholastic philosopher Al-Ghazali. This feature distinguishes it from other cosmological arguments, such as Aquinas's Second Way, which rests on the impossibility of a causally ordered infinite regress, and those of Leibniz and Samuel Clarke, which refer to the principle of sufficient reason.

The identity of indiscernibles is an ontological principle that states that there cannot be separate objects or entities that have all their properties in common. That is, entities x and y are identical if every predicate possessed by x is also possessed by y and vice versa. It states that no two distinct things (such as snowflakes) can be exactly alike, but this is intended as a metaphysical principle rather than one of natural science. A related principle is the indiscernibility of identicals, discussed below.

A form of the principle is attributed to the German philosopher Gottfried Wilhelm Leibniz. While some think that Leibniz's version of the principle is meant to be only the indiscernibility of identicals, others have interpreted it as the conjunction of the identity of indiscernibles and the indiscernibility of identicals (the converse principle). Because of its association with Leibniz, the indiscernibility of identicals is sometimes known as Leibniz's law. It is considered to be one of his great metaphysical principles, the other being the principle of noncontradiction and the principle of sufficient reason (famously used in his disputes with Newton and Clarke in the Leibniz–Clarke correspondence).

The Leibniz–Clarke correspondence was a scientific, theological and philosophical debate conducted in an exchange of letters between the German thinker Gottfried Wilhelm Leibniz and Samuel Clarke, an English supporter of Isaac Newton during the years 1715 and 1716. The exchange began because of a letter Leibniz wrote to Caroline of Ansbach, in which he remarked that Newtonian physics was detrimental to natural theology. Eager to defend the Newtonian view, Clarke responded, and the correspondence continued until the death of Leibniz in 1716.

Although a variety of subjects are touched on in the letters, the main interest for modern readers is in the dispute between the absolute theory of space favoured by Newton and Clarke, and Leibniz's relational approach. Also important is the conflict between Clarke's and Leibniz's opinions on free will and whether God must create the best of all possible worlds.