But some one may say: "I supposed that the conditions of mental activity were well known; they are quiet, peace of mind, neither too much nor too little food, and a subject which interests the feelings, or effectually calls forth the powers of the mind."
Though you know all this, you are in ignorance still. Truly a savage might profess the art of agriculture in this fashion; for all this is only as if one were to say that the conditions of success in farming were to be where there were no earthquakes or avalanches, that is, to be quiet; to have the ground cleared of trees, that is, to have the mind free from cares and the shadows of sorrows; to have neither too much nor too little sunshine and rain, that is, to be properly fed; and to have good seed to put into the ground, that is, to engage the mind with a topic which it will expand and reproduce. After all these things have been secured, it is only a sort of barbaric husbandry that we have practised. The common and rude experience of men, laboring without thinking about their labor, teaches these things, and the very beginnings of the art and science of Intellectual Economy come beyond and after these.
What shall we say of those moods which every student passes through, which turn and return upon the mind, irresistible and mysterious? What are the causes of those strange and delightful exaltations of mind in which thought runs like a clock when the pendulum is off, and crowds a week of existence into an hour of time? Whence are those dull days which come so unexpectedly, and sometimes lead a troop of dull followers, to interrupt our life's work for a week at a time? Where are we to search for obstructions in the channels of the mind when ideas will not flow? How is it that, after a period of clearness and activity in thought, the brain grows indolent, and, without a feeling of illness, or even of fatigue, work lags and stops? By what right is it that, at times, each faculty in our possession seems to grow independent, and refuses to return to its task at our call? What are the secret psychological conditions which influence the mental powers as strangely as if there were a goblin who had power to mesmerize Fancy and put it to sleep, to lock up Imagination in a dreary den of commonplaces, to blindfold Attention and make sport of his vain groping, and to send sober Reason off on foolish errands, so that Mistress Soul has not a servant left?
Such variations of mental power, which we call moods of mind, are often caused, doubtless, by ill-health, or by fatigue, or by some irregularity of habit, or by anxiety of mind; but the experience of every student will probably attest the existence of such variations where none of these causes can be assigned. There are moods which we cannot trace to illness, or weariness, or external circumstances. Men are prone to regard them as whims, which sometimes they struggle against and sometimes they yield to, but at all times wonder at.
The connection of the mind and body, and the dependence of the mind upon the health and vigor of the body, have been much dwelt upon; and we cannot be too deeply sensible of the debt which the student owes to those who have made this truth prominent to him. But, after all, it is wonderful with how much independence of bodily suffering—and even of suffering in the brain—the mind carries itself, and this fact seems worthy of more distinct recognition than it has received. It significantly confirms our belief in the existence of an immaterial principle, or soul, superior to the mere functions of the brain. Great and healthful mental activity often exists in a disordered body; and biographical literature is full of illustrations of the power of a strong will to accomplish brilliant results while the system is agitated by physical distress.
Campbell, the poet, pursued his regular habit of writing every day, even under the pressure of much bodily pain.
Cowper never, when it was possible to perform his task, excused his frail and desponding body from attendance in his little summer-house, morning and afternoon, until his forty lines of Homer were arrayed in English dress. The ballad of "John Gilpin" originated during one of his illnesses. With the hope of diverting his mind during an unusually severe attack of gloom, Lady Austen related to him the history of the renowned citizen, which she had heard in her childhood. The tale made a vivid impression, and the next morning he told her that the ludicrous incident had convulsed him with laughter during the night, and that he had embodied the whole into a ballad.
Paley's last, and perhaps, for his day, his greatest work,—his "Natural Theology,"—was principally composed during the period in which he was subject to attacks of that terribly malady, nephralgia.
So great was the delicacy of John Locke's constitution, that he was not capable of a laborious application to the medical art, which was his profession; and it is not improbable that his principal motive in studying it was that he might be qualified, when necessary, to act as his own physician. His difficulty was a lung complaint, or asthma; but his biographer says: "It occasioned disturbance to no person but himself, and persons might be with him without any other concern than that created by seeing him suffer." Notwithstanding this permanent suffering, his works are alike laborious and voluminous.
Robert Hall, in the period when his intellectual power was most vigorous, pursued his daily studies almost regardless of the pain which was his companion through life. Dr. Gregory pursued a course of study with him in metaphysics and in mathematics; and he writes: "On entering his room in the morning, I could at once tell whether or not his night had been refreshing; for if it had, I found him at the table, the books to be studied ready, and a vacant chair set for me. If his night had been restless, and the pain still continued, I found him lying on the sofa, or, more frequently, upon three chairs, on which he could obtain an easier position. At such seasons, scarcely ever did a complaint issue from his lips; but, inviting me to take the sofa, our reading commenced.... Sometimes, when he was suffering more than usual, he proposed a walk in the fields, where, with the appropriate book as our companion, we could pursue the subject. If he was the preceptor, as was commonly the case in these peripatetic lectures, he soon lost the sense of pain, and nearly as soon escaped from our author, whoever he might be, and expatiated at large upon some train of inquiry or explication which our course of reading had suggested. As his thoughts enkindled, both his steps and his words became quicker, until erelong it was difficult to say whether the body or the mind were brought most upon the stretch in keeping up with him."