Legend has it that Bricklin was watching his university professor create a table of calculation results on a blackboard. When the professor found an error, he had to tediously erase and rewrite a number of sequential entries in the table, triggering Bricklin to think that he could replicate the process on a computer, using a blackboard/spreadsheet paradigm? to view results of underlying formulas.
Later, more powerful clones of VisiCalc include Borland [Quattro Pro]?, Lotus 1-2-3 and Microsoft Excel.
External link: http://www.danbricklin.com/visicalc.htm