Nasir Khusraw was a leading Shia Ismaili poet and theologian-philosopher of the eleventh century (1004 to circa 1088 CE). In an English translation of his Gushayish wa Rahayish by Faquir Hunzai, Khusraw explains his understanding of the Siratal Mustaqim:
On the Meaning of the Straight Path
(168) O brother! You asked: 'What is the sirat (lit. path, way, bridge)? It is said that the sirat is stretched over hell, that it is thinner than an hair and sharper than a sword, and all people have to cross it. The fortunate ones cross it and reach paradise, whereas the unfortunate ones fall from it into hell. Explain, so that we may know.'
(169) Know, O brother, that (the word) sirat (in Persian, rah) means a path or a way. The path is of two kinds: one is the external path that the people walk upon the surface of the earth, and the other is the path which people follow with their souls in goodness and badness. Had the path stretched over hell been the only one which people have to cross, God in His book would not have mentioned it in the Surat al-hamd and commanded us to remember Him so that He would show us the path, as He says in the verse: 'Guide us to the straight path (al-sirat al-mustaqim)'. (1:5) Since He has commanded us to seek the straight path, it is a proof that on the path which is not straight but crooked is found that which is other than God. If God had made only one path on which we had to walk and traverse, He would not have commanded us to say this prayer. (The straight path is the way of those upon whom God has bestowed His favours, and they are the prophets,the truthful, the witnesses, and the righteous.) As He says: ('All who obey God and the Messenger are in the company of those) upon whom God has bestowed (His) favours: the prophets (nabiyyin) the truthful (siddiqin), the witnesses (shuhada), and the righteous (salihin). (4:69)
(170) Thus, it is established that the sirat is not (a path for the body) but the path of the soul which it should traverse, because God obliged (this path) first for the prophets, then their legatees (wasis) and the true Imams, and then their (proofs (hujjats)), as mentioned. These are the ones whom God has obliged: the prophets who are the Messengers, and they are so called because they convey the news of that world to the people; 'the truthful' by which He means the legatees who (expounded) the ta'wil of the shari'at and the book to the people, and by so doing disclosed the reality of the parables which they contained and provided to the wise that the Messengers are truthful; by 'the witnesses' are meant the true Imams as they are witnesses of God among the people; and by 'the righteous' are meant their (proofs) because of the betterment of the souls of people is due to them.
(171) When we come to know that the sirat is the path of the soul and not a path for the body, and with regard to what has been said that it stretches over hell, that it is thinner than an hair and sharper than a sword, that people have to traverse it in order to reach paradise, and if they fall from it they reach the eternal fire - all this is correct, but it is necessary to know its esoteric meaning (ta'wil), not (merely) the exoteric description. Thus we say that the sirat has the status of man (who is positioned) between animality and angelicity, and is required to walk on it straight because unless he traverses it he will be unable to reach paradise. Paradise is the higher (spiritual) world and hell is the fire which surrounds this lower (material) world. The ta'wil of this statement is that paradise means our liberation from the world of animality, and hell means to remain in that (animal) nature. If man practices the shari'at without understanding its ta'wil, then he makes himself into an animal, he inclines towards the left hand and falls into hell from the sirat. If he acquires (esoteric) knowledge, but does not practice the shari'at while claiming angelicity, he inclines towards the right hand and falls into hell from the sirat. However, when man walks on the path of humanity, in which he has a share from both animality and angelicity - that is, he does the work which is the share of his body and acquires knowledge which is the share of his soul - he walks on the straight path (sirat-i mustaqim); then when he traverses the sirat he is said to have reached paradise. This is so because having walked on the straight path using both knowledge and practice, when his soul leaves the body which is his sirat, he reaches the higher world, the place of angels and the true paradise." (1998, pp. 104-106).
~ Excerpted from "Knowledge and Liberation - A Treatise on Philosophical Theology" by Nasir Khusraw, edited and translated by Faquir M Hunzai.