  We present an analysis of the host properties of 85224 emission-line galaxies
from the Sloan Digital Sky Survey. We show that Seyferts and LINERs form
clearly separated branches on the standard optical diagnostic diagrams. We
derive a new empirical classification scheme which cleanly separates
star-forming galaxies, composite AGN-HII galaxies, Seyferts and LINERs and we
study the host galaxy properties of these different classes of objects. LINERs
are older, more massive, less dusty and more concentrated, and have higher
velocity dispersions and lower [OIII] luminosities than Seyfert galaxies. We
consider the quantity L[OIII]/sigma^4, which is an indicator of the black hole
accretion rate relative to the Eddington rate. Remarkably, we find that at
fixed L[OIII]/sigma^4, all differences between Seyfert and LINER host
properties disappear. LINERs and Seyferts form a continuous sequence, with
LINERs dominant at low L/L(EDD) and Seyferts dominant at high L/L(EDD). These
results suggest that the majority of LINERs are AGN and that the Seyfert/LINER
dichotomy is analogous to the high/low-state transition for X-ray binary
systems. We apply theoretical photoionization models and show that pure LINERs
require a harder ionizing radiation field with lower ionization parameter than
Seyfert galaxies, consistent with the low and high X-ray binary states.
