The tragedy of town planning is that it has, since its early days, sought to thin out the city and separate activities from each other. In order to achieve this variety of activities, you need variety of building types, a mixture of uses, blocks, building sizes, ages and conditions, types and adaptability. You also need lively uses on the ground floors, because this helps generate an active street life, can provide opportunities to people watch and help improve natural surveillance. If you are to have an active street life, you must have pedestrian flow, and therefore the concept of permeability is of key importance. People need to be able to move around places with relative ease, crossing roads, seeing around corners, being tempted down the 'side streets of disorder' as well as sticking to the 'avenues of order'.
Permeability is the capacity to move into and through an area.
One thing which destroys permeability is the construction of large blocks with dead ground floor frontages and which take up too large a footprint in the city. But of course, for people to want to move through an area, they must have things to do - which brings us back to activity and people attractors. This necessitates taking a more holistic view of cities, their culture, economy and identity. Rather than comprehensive, rational planning, many urban areas require a bit of respect, a helping hand, an injection of new money and activity. This incremental change, coupled with selective strategic interventions to effect wider changes and improvements.