The _Natural Questions_ were, as he tells us, the work of his old age.(1637) He has a lofty conception of his task, of the importance of the subject to the right culture of the spirit, and he summons up all his remaining energy to do it justice. But the work falls far short, in interest and executive skill, of a treatise like the _De Beneficiis_, and the principle of edification—_omnibus sermonibus aliquid salutare miscendum_(1638)—is too obtrusive, and sometimes leads to incongruous and almost ludicrous effects. A reference to the mullet launches him on a discourse on luxury.(1639) A discourse on mirrors would hardly seem to lend itself to moralising. Yet the invention furnishes to Seneca impressive lessons on self-knowledge, and a chance of glorifying the simple age when the unkempt daughter of a Scipio, who received her scanty dowry in uncoined metal, had never had her vanity aroused by the reflected image of her charms.(1640) The subject of lightning naturally gives occasion to a homily against the fear of death.(1641) A prologue, on the conflict to be waged with passion and luxury and chance and change, winds up abruptly with the invitation—_quaeramus ergo de aquis_ ... _qua ratione fiant_.(1642) The investigation closes with an imaginative description of the great cataclysm which is destined to overwhelm in ruin the present order. The earthquakes in Campania in 66 A.D. naturally furnish many moral lessons.(1643) The closing passage of the _Natural Questions_ is perhaps the best, and the most worthy of Seneca. In all these inquiries, he says, into the secrets of nature, we should proceed with reverent caution and self-distrust, as men veil their faces and bend in humbleness before a sacrifice.(1644) How many an orb, moving in the depths of space, has never yet risen upon the eyes of man.(1645) The Great Author Himself is only dimly visible to the inner eye, and there are vast regions of His universe which are still beyond our ken, which dazzle us by their effulgence, or elude our gross senses by their subtle secrecy. We are halting on the threshold of the great mysteries. There are many things destined to be revealed to far-distant ages, when our memory shall have passed away,(1646) of which our time does not deserve the revelation. Our energies are spent in discovering fresh ingenuities of luxury and monstrous vice. No one gives a thought to philosophy; the schools of ancient wisdom are deserted and left without a head.(1647) It is in this spirit that Seneca undertook his mission as a saviour of souls.