Fluorobenzene shows signs of both the electron withdrawing and electron donating groups on benzene. Electrophilic substitution to the ring is less reactive because the fluorine is very electronegative and withdraws electron density, but it can also form a weak conjugated system, which is especially pronounced at the ortho and para positions. Why is the modification of the sigma system (induction) related to reactivity, but modification of the pi system (conjugation) affects reactivity? What prevents it from being the other way around, where fluorobenzene is more reactive than benzene, but is not ortho-para directing? What difference in reactivity vs. selectivity makes this difference?