Board Thread:Theories and Speculation/@comment-26388300-20150612203442/@comment-3568954-20150613145249

Like itachi said all jutsu have weaknesses

that being said its not really that complicated

an edo tensei is a summoning technique one summons a soul from the pure land using the impure Resurrection technique binding that person to a body using the hand seals Snake → Ram → Boar → Dog → Tiger

if the Edo tensei shinobi knows the specific hand seals used to summon them they can take possession of the contract being the sole owner over it and as such truly immortal this is specifically what madara stated

now one could take his statements as true or one can assume that maybe he was capable of doing it because he had the power of the sage of the six paths(being that the sage himself could release an edo tensei contract)