The early years of Demosthenes's manhood were spent in preparing speeches for sale, in instructing pupils in rhetoric, and in the severe and painstaking education of himself as a public speaker. His resolution in overcoming obstacles is much dwelt upon by ancient writers. He at first lisped and stammered and had a weak voice. To cure these faults he enunciated with pebbles in his mouth and declaimed while walking uphill and by the roaring breakers of the sea-shore. He shut himself in an underground study, which he constructed for the purpose, and practised going through long trains of thought there alone. "When he went out upon a visit or received one," says Plutarch, "he would take something that passed in conversation, some business or fact that was reported to him, for a subject to exercise himself upon. As soon as he had parted from his friends, he went to his study, where he repeated the matter in order as it passed, together with the arguments for and against it. The substance of the speeches which he heard he committed to memory, and afterward reduced them to regular sentences and periods, meditating a variety of corrections and new forms of expression, both for what others had said to him and he had addressed to them. Hence it was concluded that he was not a man of much genius, and that all his eloquence was the effect of labor. A strong proof of this seemed to be that he was seldom heard to speak anything extempore, and though the people often called upon him by name as he sat in the assembly, to speak to the point debated, he would not do it unless he came prepared." It is related that when in speaking he happened to be thrown into confusion by any occurrence in the assembly, the orator Demades, the foremost extempore speaker of the age, often arose and supported him in an extempore address, but that he never did this for Demades. Demosthenes was not, however, the slave of manuscript or memory. He declared that "he neither wrote the whole of his orations nor spoke without first committing part to writing." There was said to be greater spirit and boldness in his impromptu speeches than in those which he had elaborately prepared. People thought that sometimes when he spoke out thus on a sudden, his eloquence was inspired from above, as when once he uttered, in regular though unpremeditated verse, the forceful oath: "By earth, by all her fountains, streams, and floods."