“When one begins to work in order to bestow, and this goes against ones nature, this is called "toil," since the body (refers to ones wills and desires) opposes it, being that any movement which the body does not see that it will be use for itself, it opposes with all of its strength, and tremendous powers are needed to overcome it.”

A note on this statement is included by Stillman Drake in his Galileo at Work, His Scientific Biography (1981): Galileo adhered to this position in his Dialogue at least as to the "integral bodies of the universe." by which he meant stars and planets, here called "parts of the universe." But he did not attempt to explain the planetary motions on any mechanical basis, nor does this argument from "best arrangement" have any bearing on inertial motion, which to Galileo was indifference to motion and rest and not a tendency to move, either circularly or straight.
Letter to Francesco Ingoli (1624)

Pharmacokinetics: Basic Principles and Its Use as a Tool in Drug Metabolism, p. 199 in Drug Metabolism and Drug Toxicity, Mitchell JR, Horning MG, editors, Raven Press, New York, 1984.

First Tractate : The Animate and the Man, §3
The First Ennead (c. 250)

4 : Unification of mind & body
Ki Sayings (2003)

"Excerpts from the Teaching of Hans Hofmann", p. 66
Search for the Real and Other Essays (1948)

Letter 8; Variant: The greater part of men are much too exhausted and enervated by their struggle with want to be able to engage in a new and severe contest with error. Satisfied if they themselves can escape from the hard labour of thought, they willingly abandon to others the guardianship of their thoughts.
On the Aesthetic Education of Man (1794)
Context: Dare to be wise! Energy and spirit is needed to overcome the obstacles which indolence of nature as well as cowardice of heart oppose to our instruction. It is not without significance that the old myth makes the goddess of Wisdom emerge fully armed from the head of Jupiter; for her very first function is warlike. Even in her birth she has to maintain a hard struggle with the senses, which do not want to be dragged from their sweet repose. The greater part of humanity is too much harassed and fatigued by the struggle with want, to rally itself for a new and sterner struggle with error. Content if they themselves escape the hard labor of thought, men gladly resign to others the guardianship of their ideas, and if it happens that higher needs are stirred in them, they embrace with a eager faith the formulas which State and priesthood hold in readiness for such an occasion.

Idea for a Universal History from a Cosmopolitan Point of View (1784)
Context: Reason in a creature is a faculty of widening the rules and purposes of the use of all its powers far beyond natural instinct; it acknowledges no limits to its projects. Reason itself does not work instinctively, but requires trial, practice, and instruction in order gradually to progress from one level of insight to another. Therefore a single man would have to live excessively long in order to learn to make full use of all his natural capacities. Since Nature has set only a short period for his life, she needs a perhaps unreckonable series of generations, each of which passes its own enlightenment to its successor in order finally to bring the seeds of enlightenment to that degree of development in our race which is completely suitable to Nature’s purpose. This point of time must be, at least as an ideal, the goal of man’s efforts, for otherwise his natural capacities would have to be counted as for the most part vain and aimless. This would destroy all practical principles, and Nature, whose wisdom must serve as the fundamental principle in judging all her other offspring, would thereby make man alone a contemptible plaything.

Second Thesis
Paraphrased variant: Reason does not work instinctively, but requires trial, practice, and instruction in order to gradually progress from one level of insight to another.

Upon the Sovereign Sun (362)

Source: Books, Spiritual Warrior, Volume I: Uncovering Spiritual Truths in Psychic Phenomena (Hari-Nama Press, 1996), Chapter 1: Dreams: A State of Reality, p. 20