The distance is commonly very great between actual performances and speculative possibility. It is natural to suppose, that as much as has been done to-day may be done to-morrow; but on the morrow some difficulty emerges, or some external impediment obstructs. Indolence, interruption, business, and pleasure, all take their turns of retardation; and every long work is lengthened by a thousand causes that can, and ten thousand that cannot, be recounted. Perhaps no extensive and multifarious performance was ever effected within the term originally fixed in the undertaker’s mind. He that runs against Time, has an antagonist not subject to casualties.

Samuel Johnson (1709-1784) English writer, lexicographer, critic

Lives of the Most Eminent English Poets, “Pope” (1781)
    (Source)

Also known as Lives of English Poets and Lives of the Poets.