I have always been a very great Lover of your Speculations, as well in Regard to the Subject, as to your Manner of Treating it. Human Nature I always thought the most useful Object of human Reason, and to make the Consideration of it pleasant and entertaining, I always thought the best Employment of human Wit: Other Parts of Philosophy may perhaps make us wiser, but this not only answers that End, but makes us better too. Hence it was that the Oracle pronounced Socrates the wisest of all Men living, because he judiciously made Choice of human Nature for the Object of his Thoughts; an Enquiry into which as much exceeds all other Learning, as it is of more Consequence to adjust the true Nature and Measures of Right and Wrong, than to settle the Distance of the Planets, and compute the Times of their Circumvolutions.
One good Effect that will immediately arise from a near Observation of human Nature, is, that we shall cease to wonder at those Actions which Men are used to reckon wholly unaccountable; for as nothing is produced without a Cause, so by observing the Nature and Course of the Passions, we shall be able to trace every Action from its first Conception to its Death; We shall no more admire at the Proceedings of Catiline or Tiberius, when we know the one was actuated by a cruel Jealousie, the other by a furious Ambition; for the Actions of Men follow their Passions as naturally as Light does Heat, or as any other Effect flows from its Cause; Reason must be employed in adjusting the Passions, but they must ever remain the Principles of Action.
The strange and absurd Variety that is so apparent in Men's Actions, shews plainly they can never proceed immediately from Reason; so pure a Fountain emits no such troubled Waters: They must necessarily arise from the Passions, which are to the Mind as the Winds to a Ship, they only can move it, and they too often destroy it; if fair and gentle, they guide it into the Harbour; if contrary and furious, they overset it in the Waves: In the same manner is the Mind assisted or endangered by the Passions; Reason must then take the Place of Pilot, and can never fail of securing her Charge if she be not wanting to her self: The Strength of the Passions will never be accepted as an Excuse for complying with them, they were designed for Subjection, and if a Man suffers them to get the upper Hand, he then betrays the Liberty of his own Soul.
As Nature has framed the several Species of Beings as it were in a Chain, so Man seems to be placed as the middle Link between Angels and Brutes: Hence he participates both of Flesh and Spirit by an admirable Tie, which in him occasions perpetual War of Passions; and as a Man inclines to the angelick or brute Part of his Constitution, he is then denominated good or bad, virtuous or wicked; if Love, Mercy, and Good-nature prevail, they speak him of the Angel; if Hatred, Cruelty, and Envy predominate, they declare his Kindred to the Brute. Hence it was that some of the Ancients imagined, that as Men in this Life inclined more to the Angel or Brute, so after their Death they should transmigrate into the one or the other: and it would be no unpleasant Notion, to consider the several Species of Brutes, into which we may imagine that Tyrants, Misers, the Proud, Malicious, and Ill-natured might be changed.
As a Consequence of this Original, all Passions are in all Men, but all appear not in all; Constitution, Education, Custom of the Country, Reason, and the like Causes, may improve or abate the Strength of them, but still the Seeds remain, which are ever ready to sprout forth upon the least Encouragement. I have heard a Story of a good religious Man, who, having been bred with the Milk of a Goat, was very modest in Publick by a careful Reflection he made on his Actions, but he frequently had an Hour in Secret, wherein he had his Frisks and Capers; and if we had an Opportunity of examining the Retirement of the strictest Philosophers, no doubt but we should find perpetual Returns of those Passions they so artfully conceal from the Publick. I remember Matchiavel observes, that every State should entertain a perpetual jealousie of its Neighbours, that so it should never be unprovided when an Emergency happens;  in like manner should the Reason be perpetually on its Guard against the Passions, and never suffer them to carry on any Design that may be destructive of its Security; yet at the same Time it must be careful, that it don't so far break their Strength as to render them contemptible, and consequently it self unguarded.
The Understanding being of its self too slow and lazy to exert it self into Action, its necessary it should be put in Motion by the gentle Gales of the Passions, which may preserve it from stagnating and Corruption; for they are as necessary to the Health of the Mind, as the Circulation of the animal Spirits is to the Health of the Body; they keep it in Life, and Strength, and Vigour; nor is it possible for the Mind to perform its Offices without their Assistance: These Motions are given us with our Being, they are little Spirits that are born and dye with us; to some they are mild, easie, and gentle, to others wayward and unruly, yet never too strong for the Reins of Reason and the Guidance of Judgment.
We may generally observe a pretty nice Proportion between the Strength of Reason and Passion; the greatest Genius's have commonly the strongest Affections, as on the other hand, the weaker Understandings have generally the weaker Passions; and 'tis fit the Fury of the Coursers should not be too great for the Strength of the Charioteer. Young Men whose Passions are not a little unruly, give small Hopes of their ever being considerable; the Fire of Youth will of course abate, and is a Fault, if it be a Fault, that mends every Day; but surely unless a Man has Fire in Youth, he can hardly have Warmth in Old Age. We must therefore be very cautious, lest while we think to regulate the Passions, we should quite extinguish them, which is putting out the Light of the Soul: for to be without Passion, or to be hurried away with it, makes a Man equally blind. The extraordinary Severity used in most of our Schools has this fatal Effect, it breaks the Spring of the Mind, and most certainly destroys more good Genius's than it can possibly improve. And surely 'tis a mighty Mistake that the Passions should be so intirely subdued; for little Irregularities are sometimes not only to be borne with, but to be cultivated too, since they are frequently attended with the greatest Perfections. All great Genius's have Faults mixed with their Virtues, and resemble the flaming Bush which has Thorns amongst Lights.
Since, therefore the Passions are the Principles of human Actions, we must endeavour to manage them so as to retain their Vigour, yet keep them under strict Command; we must govern them rather like free Subjects than Slaves, lest while we intend to make them obedient, they become abject, and unfit for those great Purposes to which they were designed. For my Part I must confess, I could never have any Regard to that Sect of Philosophers, who so much insisted upon an absolute Indifference and Vacancy from all Passion; for it seems to me a Thing very inconsistent for a Man to divest himself of Humanity, in order to acquire Tranquility of Mind, and to eradicate the very Principles of Action, because its possible they may produce ill Effects.
I am, SIR,
Your Affectionate Admirer,
[Footnote 1: The Prince, ch. xlv, at close.]Translation of motto: