This book, together with the next two volumes to follow, offers the scientific community the works and thoughts of Santiago Ramón y Cajal. The text is a faithful rendition of the original Spanish version, with additional facts taken from the French translation. Both of these are currently quoted an average of 200 times a year in the scientific literature. This collection will represent the "definitive Cajal" for scientists and scholars interested in the original thoughts of probably the most prominent neuroscientist of all time.