Some say that a paragraph should have (only) one idea (or subject or topic). In that case, what constitutes an idea (or subject or topic)? And how might one verify that a putative paragraph possesses but one idea?
I will put forth two examples, for the purposes of discussion. The first, a one-sentence paragraph Carroll might have written:
"What a curious feeling—I must be shutting up like a telescope."
And another thing (it looks like a paragraph), of Swift:—
It would not be proper, for some reasons, to trouble the reader with the particulars of our adventures in those seas; let it suffice to inform him, that in our passage from thence to the East Indies, we were driven by a violent storm to the northwest of Van Diemen’s Land. By an observation, we found ourselves in the latitude of 30 degrees 2 minutes south. Twelve of our crew were dead by immoderate labor and ill food: the rest were in a very weak condition. On the 5th of November, which was the beginning of summer in those parts, the weather being very hazy, the seamen spied a rock within half a cable’s length of the ship, but the wind was so strong that we were driven directly upon it, and immediately split. Six of the crew, of whom I was one, having let down the boat into the sea, made a shift to get clear of the ship and the rock. We rowed, by my computation, about three leagues, till we were able to work no longer, being already spent with labor while we were in the ship. We therefore trusted ourselves to the mercy of the waves, and in about half an hour the boat was overset by a sudden flurry from the north. What became of my companions in the boat, as well as of those who escaped on the rock, or were left in the vessel, I cannot tell, but conclude they were all lost. For my own part, I swam as fortune directed me, and was pushed forward by wind and tide. I often let my legs drop, and could feel no bottom, but when I was almost gone, and able to struggle no longer, I found myself within my depth; and by this time the storm was much abated. The declivity was so small, that I walked near a mile before I got to the shore, which I conjectured was about eight o’clock in the evening. I then advanced forward near half a mile, but could not discover any sign of houses or inhabitants; at least I was in so weak a condition that I did not observe them. I was extremely tired, and with that, and the heat of the weather, and about half a pint of brandy that I drank as I left the ship, I found myself much inclined to sleep. I lay down on the grass, which was very short and soft, where I slept sounder than ever I remember to have done in my life, and, as I reckoned, about nine hours; for when I awakened, it was just daylight. I attempted to rise, but was not able to stir; for, as I happened to lie on my back, I found my arms and legs were strongly fastened on each side to the ground; and my hair, which was long and thick, tied down in the same manner. I likewise felt several slender ligatures across my body, from my arm-pits to my thighs. I could only look upwards; the sun began to grow hot, and the light offended my eyes. I heard a confused noise about me; but in the posture I lay, could see nothing except the sky. In a little time, I felt something alive moving on my left leg, which advancing gently forward over my breast, came almost up to my chin; when, bending my eyes downwards as much as I could, I perceived it to be a human creature not six inches high, with a bow and an arrow in his hands and a quiver at his back. In the meantime I felt at least forty more of the same kind (as I conjectured) following the first. I was in the utmost astonishment, and roared so loud that they all ran back in a fright, and some of them, as I was afterwards told, were hurt by the falls they got by leaping from my sides upon the ground. However, they soon returned, and one of them who ventured so far as to get a full sight of my face, lifting up his hands and eyes by way of admiration, cried out in a shrill but distinct voice, Hekinah degul! The others repeated the same words several times, but I then knew not what it meant. I lay all this while, as the reader may believe, in great uneasiness; at length, struggling to get loose, I had the fortune to break the strings, and wrench out the pegs that fastened my left arm to the ground, for, by lifting it up to my face, I discovered the methods they had taken to bind me, and at the same time with a violent pull, which gave me excessive pain, I a little loosened the strings that tied down my hair on the left side, so that I was just able to turn my head about two inches. But the creatures ran off a second time, before I could seize them; whereupon there was a great shout in a very shrill accent, and after it had ceased I heard one of them cry aloud, Tolgo phonac; when in an instant I felt above a hundred arrows discharged on my left hand, which pricked me like so many needles; and besides they shot another flight into the air, as we do bombs in Europe, whereof many, I suppose, fell on my body (though I felt them not) and some on my face, which I immediately covered with my left hand. When this shower of arrows was over, I fell a-groaning with grief and pain, and then striving again to get loose, they discharged another volley larger than the first, and some of them attempted with spears to stick me in the sides; but by good luck I had on me a buff jerkin, which they could not pierce. I thought it the most prudent method to lie still, and my design was to continue so till night, when, my left hand being already loose, I could easily free myself: and as for the inhabitants, I had reason to believe I might be a match for the greatest army they could bring against me, if they were all of the same size with him that I saw. But fortune disposed otherwise of me. When the people observed I was quiet, they discharged no more arrows; but, by the noise I heard, I knew their numbers increased; and about four yards from me, over against my right ear, I heard a knocking for above an hour, like that of people at work; when turning my head that way, as well as the pegs and strings would permit me, I saw a stage erected about a foot and a half from the ground, capable of holding four of the inhabitants, with two or three ladders to mount it: whence one of them, who seemed to be a person of quality, made me a long speech, whereof I understood not a syllable. But I should have mentioned, that before the principal person began his oration, he cried out three times, Langro dehul san (these words and the former were afterwards repeated and explained to me). Whereupon, immediately about fifty of the inhabitants came and cut the string that fastened the left side of my head, which gave me the liberty of turning it to the right, and of observing the person and gesture of him that was to speak. He appeared to be of middle age, and taller than any of the other three who attended him, whereof one was a page that held up his train, and seemed to be somewhat longer than my middle finger; the other two stood one on each side to support him. He acted every part of an orator, and I could observe many periods of threatenings, and others of promises, pity, and kindness. I answered in a few words, but in the most submissive manner, lifting up my left hand and both my eyes to the sun, as calling him for a witness; and being almost famished with hunger, not having eaten a morsel for some hours before I left the ship, I found the demands of nature so strong upon me that I could not forbear showing my impatience (perhaps against the strict rules of decency), by putting my finger frequently to my mouth, to signify that I wanted food. The hurgo (for so they call a great lord, as I afterwards learned) understood me very well. He descended from the stage, and commanded that several ladders should be applied to my sides, on which about a hundred of the inhabitants mounted, and walked towards my mouth, laden with baskets full of meat, which had been provided and sent thither by the king’s orders, upon the first intelligence he received of me. I observed there was the flesh of several animals, but could not distinguish them by the taste. There were shoulders, legs and loins, shaped like those of mutton, and very well dressed, but smaller than the wings of a lark. I ate them by two or three at a mouthful, and took three loaves at a time about the bigness of musketballs. They supplied me as fast as they could, showing a thousand marks of wonder and astonishment at my bulk and appetite.