Camille Pisarro, in 1896, explained to an aspiring young painter that 'precise drawing is dry and hampers the impression of the whole'. In other words 'loosen up'. He continues 'Do not define too closely the outline of things; it is the brushstroke of the right value and colour which should produce the drawings'
Of course, the method of painting aimed for was loose and not linear and cannot apply to every single object. However, ridding ourselves of the idea encouraged in childhood that we must first draw an outline and then colour it in staying within the lines loosens the finished work. By working alternately and simultaneously on the two elements, the background and foreground, creates a tension and gives the correct amount of contrast between the two.
The edge of something is simply the place at which the thing behind it becomes visible. In a painting that edge does not have to be too clearly defined - it can be more fun to to make the brain work out where one thing finishes and another starts. Allow the paint to guide the eye towards interpretation. Learn this lesson and your work will be given a freedom of movement as never before and the viewer will enjoy fitting the pieces together like a jigsaw.