ally , ▁for ▁five ▁reasons : ▁ ▁The ▁theories ▁are ▁very ▁abstract , ▁making ▁it ▁difficult ▁to ▁test ▁them ▁convin cing ly . ▁Even ▁if ▁reliable ▁data ▁is ▁available , ▁the ▁data ▁can ▁be ▁read ▁in ▁different ▁ways . ▁There ▁exist ▁a ▁very ▁large ▁number ▁of ▁different ▁factors ▁which ▁can ▁influence ▁business ▁profit ability , ▁invest ments ▁and ▁market ▁sales , ▁while ▁it ▁is ▁difficult ▁to ▁prove ▁how ▁these ▁factors ▁are ▁all ▁related , ▁or ▁to ▁prove ▁which ▁ones ▁are ▁the ▁most ▁important ▁ones ▁in ▁an ▁overall ▁sense ▁( since ▁different ▁kinds ▁of ▁business ▁operate ▁in ▁different ▁circumstances ). ▁The ▁final ▁causes ▁of ▁cr ises ▁might ▁not ▁be ▁exactly ▁the ▁same ▁in ▁every ▁crisis ▁occurr ing ▁in ▁the ▁last ▁two ▁centuries , ▁except ▁if ▁particular ▁causes ▁are ▁accepted ▁as ▁the ▁main ▁ones ▁" by ▁definition ". ▁Marx ' s ▁ideas ▁about ▁cr ises ▁were ▁based ▁on ▁the ▁kind ▁of ▁capital ism ▁that ▁existed ▁in ▁the ▁mid - 1 9 th ▁century , ▁without ▁it ▁being ▁very ▁clear ▁what ▁the ▁continu ities ▁and ▁dis continu ities ▁are ▁with ▁present - day ▁capital ism . ▁ ▁According ▁to ▁a ▁popular ▁Marx ist ▁interpretation , ▁cr ises ▁are ▁the ▁necessary ▁result ▁of ▁the ▁falling ▁profit ability ▁of ▁production ▁capital , ▁which , ▁according ▁to ▁Marx , ▁was ▁an ▁effect ▁of ▁rising ▁overall ▁product ivity ▁( ra ising ▁the ▁organ ic ▁composition ▁of ▁production ▁capital ▁and ▁lower ing ▁the ▁value ▁of ▁comm od ities ). ▁But ▁supp osing ▁that ▁we ▁can ▁prove ▁definitely ▁that ▁profit ability ▁did