In his Gifford Lectures, Stirling says his objective is to fulfil Lord Gifford’s own religious aim – to seek proofs of the existence of God. Stirling starts his lectures by defining Natural Theology as being precisely this task – to “know” religion: “We not only feel, we know, religion. Religion is not only buoyed up on a sentiment of the heart, it is founded also on ideas of the intellect.” For this reason, Stirling holds in high regard Kant’s belief in the existence of à priori truths. Contrary to Locke’s idea of a tabula rasa, Stirling argues, the existence of such truths means that every human being is born with an innate knowledge of God already embedded in their minds. This is why, so often in history, he explains, people point to the beauties of nature and conclude there must be a beautiful mind underlying such harmonies: “To any one who will approach to look, an eye, an ear is as much a necessity in the realization, is as much involved in the very plan of the universe, as matter and molecules, and the immensity of space itself. But the moment we see that, we see design also,” he writes in his fifth lecture. “We see that intelligence has gone to the composition of the universe. We have become sober, like Anaxagoras in the midst of inebriates, and like him we proclaim the nous,” where “nous” is the Greek word referent to the One, the Unity, or the Idea that underlies all the particularities and differences we perceive in our daily world. Throughout his lectures, Stirling uses this classical metaphor of the “nous” to signify an infinite, orderly mind – namely, God.

Stirling’s focus throughout the lectures is on the “argument from Design”, this being the notion that underlying the order and harmony we perceive in the empirical world there also must exist an orderly and designing mind (i.e. God). Stirling traces this argument from Anaxagoras to Charles Darwin, saving his most severe criticisms for David Hume and Darwin, two authors who he felt had failed to recognize the causal role “design” plays in nature and speciation.

In particular Stirling focuses on Hume’s attack of “inference” as it relates to Natural Theology. Hume questions the manner in which we often link “effects” and their “causes”; he declares we cannot determine the cause of any phenomenon merely from its effect. “Effects” only tell us about themselves. To step backward and infer the cause of any particular phenomenon involves invoking a metaphysical presupposition or conventional belief rather than a logical step.

Stirling says such a criticism misses the point. The “cause” of nature itself (of all the phenomena we find in the world) must be an eternal, universal God because if we think of such a God, then there must be such a God. Stirling calls this the “ontological argument” in which “the idea of infinite perfection implies that of actual existence.” This means that our capacity to conceive an infinite and perfectly designing mind means it likely exists, because that capacity is the result of being born with à priori knowledge of God already embedded in our minds. In his time, Hume had ridiculed this so-called “ontological argument” by saying that whatever we could imagine to exist, we could also imagine not to exist. There is no necessary link between our thoughts and the world as it exists. For Hume, our thoughts and ideas about the world come from our empirical interaction with it – not from à priori notions we are born with.

Yet Stirling insists Hume is wrong. It is not the case that “thought follows matter,” as Hume would have it, but rather that “matter follows thought”. “Hold up an original matter when you may, you will never hold it up without an original form; which original form too, is the original firs and furrow of the whole business,” he writes. “…At all events, we are evidently under no necessity to conclude with Hume or his belated followers that matter is, in any respect, earlier than form.” For Stirling, the sheer magnificence of the world necessarily leads us to believe something greater is at play than merely the indeterminate interaction of molecules and atoms: “Can such effect as that, the universe namely, not warrant every supremacy that we name God?” That said, Stirling claims Hume never really believed his own criticisms of “design”. Although Hume hated superstition, Stirling argues, “no thought lay nearer his heart all his life than the thought of God.”

For this reason Stirling harbours his most caustic resentment toward Darwin’s anti-theistic theory of natural selection. Darwin wants to be the Newton of his time, Stirling laments; the result is that the naturalist has failed to identify the role Idea, or Design, plays in the evolution of life. For Stirling, it is not enough to say, as Darwin does, that variation is the quintessential element necessary for evolution to take place. “It is impossible for me to accept the theory which Mr. Darwin offers,” Stirling writes in 18th lecture. “…He offers us, instead, a mechanical pullulation of individual difference which is to eventuate in all the beautiful and complicated forms, whether of plants of animal, which we see around us.” Stirling lambastes Darwin’s belief that the development of species is the result of natural laws which guide natural selection in the same way the law of gravity guides the orbit of the Earth about the sun.

In his final tirade Stirling introduces Hegel into his own Design arguments. He uses – what can only be deemed to be – an explicitly Christian interpretation of Hegel’s thought to explain that all the variation (of species) we observe in the world is the result of imperfect images of the Idea (or God’s plan). Those species are the manifestation of the Christian Fall – corporeal ideas that are constantly ameliorating themselves in a dialectical effort to regain the perfection they lost by becoming real, rather than remaining part of the ideal mind of God. This is why Stirling writes: “Nature is but the phenomenon of the noumenon, the many of the one, the externale of the internale, thrown down from the unity of reasoned co-articulation and connectedness – thrown down and abroad into the infinitude of a disunited, disconnected, disarticulated inorganic chaos, which, however turns upon itself – turns upon itself for restoration and return to the image from which it fell.”

Stirling, therefore, does not deny variation exists in nature; he simply denies it is the mere result of a series of accidental adaptations or molecular changes. Another, more theistically-minded, theory is needed to explain the variation we see, he concludes, and whatever that theory ends up being, “We claim Design for nature, whatever we admit!”