The expression “to start from scratch” means: To begin any enterprise, investigation, search, or other activity from the very beginning, often with no precedent as a guide; to begin at the beginning; to take the first step.
Actually the expression is derived from the sporting world, from a race in which scratch designates the line or mark that is to be the starting point.
Hence, he who starts from scratch starts from nothing; he has no preliminary impetus beyond his own ability, genius, or determination to carry him through the race.