Moses Mendelssohn was not a rationalist. He put much less faith in rationalism and reason than did Maimon or Kant and Hegel, who left him in the historical dustbin. Less a philosopher, Mendelssohn may have been less of a philosopher for it. Typically identified as popular philosophy, Mendelssohn’s should be better understood as aesthetic philosophy –open to and even trusting of appearance and the senses, to images and representations.
The contention that Morning Hours was a last ditch attempt to use the ontological argument to prove the “existence” of God is a lazy one that misses what more to the point and truly marvelous about the text. At issue is less the “existence” of God, although “existence” is a proper translation of Dasein, the word used by Mendelssohn. More to the point, as I would see it, is “actuality,” i.e. the actuality of God and the actuality of the existence of any contingent thing’s being, including the human subject, from whose actuality Mendelssohn starts. Unlike a necessary being, whose non-existence is inconceivable according to Mendelssohn, following the ontological argument, a contingent thing’s non-existence, including my own non-existence, is always conceivable.
There a number of steps here which I would identify as such.
 So Mendelssohn starts with the world of appearances and representations of appearances, dreams, thoughts, feelings, and sensations, assuming against the materialist science of his day, that these constitute more than “mere illusion,” as much as the problem of illusion is bound up with the phenomena of appearance. Because the existence of an appearance is at least certain qua appearance, the fact that it appears. And the appearance with which Mendelssohn starts is my own appearance as a subject, because it is one’s own actuality about which one can be most certain. The form of human consciousness with which Mendelssohn starts is not, however, a disembodied Cartestian knowing cogito. “I think therefore I am,” but according to Mendelssohn, “the philosopher could have with equal justification said I hope, therefore I am; I fear therefore I am; and so on” (38).
 The sense of each appearance as it appears to the human subject is caught up in a web, the world of common-sense, in which all the senses, representations, and appearances, are brought together to work together in tandem. The senses, for instance, the sense of sight, are most easily deceived when isolated, whereas illusions are generally dispelled when they are brought together as a whole, for instance, sight in combination with touch. Mendelssohn is an associational thinker whose very notion of perfection rests on the harmony or synthesis of all things together. All the senses working together confirm the reality or actuality of an object, person, or world outside us. Common-sense philosophy, understood as such, as rooted in the senses, has something of what Peter Berger would have called a social “plausibility structure.” It depends upon the rather rough agreement among rational and even super-rational beings with unimpaired sense regarding the actuality of said external objects, the widespread agreement across a manifold of creatures.
 The existence of the world is brought under the faculty of “approbation.” In the middle between the faculties of cognition and desire, approbation refers to the approval by which a subject regards an object, brings it into existence, or modifies its appearance and being according to an intentional purpose. Mendelssohn links it explicitly to the imagination in relation to practical (ethical) will, altered in time, whose purpose is the good, not truth per se (56). In other words, the world exists as the object of a subject’s approbation.
 The actuality of God as understood by Mendelssohn has more to do with a sense than with an actual proof. The world is perceived and conceived of as an object of God’s approbation. As Necessary Being, Mendelssohn “knew” that God could not not exist. But if the world does not exist than God cannot exist as its necessary ground or principle of sufficient reason. And this is the argument with Spinoza, who, as Mendelssohn read him, rejected the notion that “I” the human subject am distinct from the world and distinct from God as a separate substance. I am an object of God’s approval or approbation. For Mendelssohn, I think, God exists as the necessary being who approves of my existence and therefore brought it into being for a good purpose.
 What is unique to Mendelssohn is the pronounced sense of what gets called “creature consciousness” in early 20th century German theology. The creature is the actual and existing contingent being or thing whose non-existence is not just conceivable but distinctly possible and soon to be actualized. I exist as God’s intentional object (85-9). I am an object of God’s approbation, brought into the world and chosen for a purpose. What innocent self-aware sense of marvelous self-possession was Mendelssohn’s. With no sense of sin or guilt, his was a self-aware self-possession. The one is brought into relation, the senses brought into relation with other senses, the self in relation to and with other human subjects, human subjects and their feelings, imaginings and ideas in relation to the world, and me in my own existence in relation to God as an object of God’s own pleasure. I am a relatively perfect creature, not just the notion of my existence but my existence as actualized (88). Belonging to God, I exist as an idea in God’s mind always and as an actual object in the finite and actual time and place approved by God. But God belongs as much to me as I to God. Qua finite creature whose very own self-knowledge is incomplete, I am marked off and distinct, contra Spinoza, even from the image of me in God’s mind (104). This is a peculiar version of the ontological argument, the logical conclusion of which is one that I don’t think Mendelssohn would have drawn himself. The certainty is based on the sense or feeling that all of this is for the good as understood in the complete understanding of the mind of God, that if we can say that I exist, then God must exist apart from and outside me. There must be some greater being who thinks me as idea and brings me into being as actual being. So if I exist, then God must exist.
(Kudos to Bruce Rosenstock. Anglophone Jewish philosophy owes a huge debt to him for translating Morning Hours and To the Friends of Lessing, the late and last works of Mendelssohn. There’s now no excuse not to read them.)