Ramban explains that the blessings and curses of Bechukotai are miracles hidden within the natural order (nissim nistarin). He argues that the Torah is promising supernatural reward and punishment cloaked in natural appearances, revealing God's ongoing providence over Israel.
וְהַטַּעַם בָּזֶה כִּי הַבְּרָכוֹת הָהֵם אע"פ שֶׁהֵם נִסִּים הֵם מִן הַנִּסִּים הַנִּסְתָּרִים שֶׁכָּל הַתּוֹרָה מְלֵאָה מֵהֶם כַּאֲשֶׁר פֵּרַשְׁתִּי (בראשית יז א), וְהֵם אֲפִלּוּ לְיָחִיד הָעוֹבֵד, כִּי כַּאֲשֶׁר יִהְיֶה הָאִישׁ הֶחָסִיד שׁוֹמֵר כָּל מִצְוֹת ה' אֱלֹהָיו, יִשְׁמְרֵהוּ הָאֵל מִן הַחֹלִי וְהָעֲקָרוּת וְהַשְּׁכוֹל וִימַלֵּא יָמָיו בְּטוֹבָה. אֲבָל אֵלּוּ הַבְּרָכוֹת שֶׁבַּפָּרָשָׁה הַזֹּאת הֵן כְּלָלִיּוֹת בָּעָם, וְהֵן בִּהְיוֹת כָּל עַמֵּנוּ כֻּלָּם צַדִּיקִים. וּכְבָר בֵּאַרְנוּ כִּי כָּל אֵלֶּה הַבְּרָכוֹת כֻּלָּם נִסִּים, אֵין בַּטֶּבַע שֶׁיָּבֹאוּ הַגְּשָׁמִים וְיִהְיֶה הַשָּׁלוֹם לָנוּ מִן הָאוֹיְבִים וְיָבֹא מֹרֶךְ בְּלִבָּם לָנוּס מֵאָה מִפְּנֵי חֲמִשָּׁה בַּעֲשׂוֹתֵנוּ הַחֻקִּים וְהַמִּצְוֹת, וְלֹא שֶׁיִּהְיֶה הַכֹּל הֵפֶךְ מִפְּנֵי זָרְעֵנוּ הַשָּׁנָה הַשְּׁבִיעִית. וְאַף עַל פִּי שֶׁהֵם נִסִּים נִסְתָּרִים שֶׁעוֹלָם כְּמִנְהָגוֹ נוֹהֵג עִמָּהֶם, אֲבָל הֵם מִתְפַּרְסְמִים מִצַּד הֱיוֹתָם תָּמִיד לְעוֹלָם בְּכָל הָאָרֶץ, כִּי אִם הַצַּדִּיק הָאֶחָד יִחְיֶה וְיָסִיר ה' מַחֲלָה מִקִּרְבּוֹ וִימַלֵּא יָמָיו, יִקְרֶה גַּם זֶה בִּקְצָת רְשָׁעִים
For there He assured them that their food and drink would be [a source of] blessing, so that no sickness will occur to our bodies, and therefore the organs of procreation will be complete and healthy and we will give birth properly, and live full lives, as He said, None shall miscarry, nor be barren in thy Land, the number of thy days will I fulfill, and similarly He said at the beginning for I am the Eternal that healeth thee. The reason for this [division of the blessings into two sections] is that [all] the blessings [mentioned in the Torah], although they are miraculous, are hidden miracles of which the whole Torah is full, as I have explained. They apply even to an individual who serves [Him], for when there is a pious man who keeps all the commandments of his G-d, the Eternal G-d will guard him from sickness, barrenness and bereavement, and he will live out his days in goodness. But these blessings which are stated in this section are general ones applying to the people [as a whole], and they [take effect] when the whole of our people shall all be righteous. Therefore He mentions here always the Land: and the Land shall yield; in your Land safely; peace in the Land; out of the Land; it shall not go through your Land. Now we have already explained that all these blessings are miracles, for it is not natural that the rains should come [in their due season], and that we should have peace from our enemies, and that they should have faintness of heart so that a hundred of them flee before five, as a result of us observing the statutes and commandments of G-d, nor that everything should be the opposite [i.e.