Regarding whether or not it is our duty to rationally investigate on the unity of G-d, I will say as follows: For anyone who is capable of investigating on this and other similar matters through rational inquiry - it is his duty to do so according to his intelligence and perception.
I have already written in the introduction to this book sufficient arguments which demonstrate the obligation of this matter. Anyone who neglects to investigate into it is blameworthy and is considered as belonging to the class of men who fall short in wisdom and conduct. He is like a sick man (a doctor) who is an expert on the nature of his disease and the correct healing method, but instead relies on another doctor to heal him who applies various healing methods, while he is lazy to inquire using his own wisdom and reasoning into the methods employed by the doctor, to see whether or not the doctor is dealing with him correctly or not, when he was easily able to do this without anything preventing him. The Torah has already obligated us on this, as written: "know therefore today, and lay it to your heart[, that the L-ord is G-d in heaven above and on the earth beneath; there is no other]" (Deut. 4:39).
The proof that "lay it to your heart" refers to intellectual investigation, is from what the following verse says: "And none lays it to his heart, neither is there knowledge nor understanding" (Isaiah 44:19). So too David urged his son: "And you, Solomon my son, know you the G-d of your father, and serve him with a perfect heart and with a willing soul; for the L-ord searches all hearts" (I Chronicles 28:9).
And "Because he has set his love upon Me, therefore will I deliver him: I will set him on high, because he has known My Name" (Ps. 91:14), and "But let him that glories glory in this, that he understands and knows Me" (Yirmiya 9:23), and our Sages said: "be diligent in the study of Torah and know what to answer a heretic" (Avos 2:14), and the Torah says: "Keep therefore and do them; for this is your wisdom and your understanding in the sight of the nations.." (Deut. 4:6).
And it is impossible for the nations to admit to our claims of superior wisdom and understanding unless there are proofs and evidences which can testify for us along with the testimony of the intellect on the truth of our Torah and our faith. And our Maker has already promised us that He will remove the veil of ignorance from their minds, and show His magnificent glory as a sign to us on the truth of our Torah when He said: "And the nations shall walk by your light" (Isaiah 60:3), and "And many peoples shall go and say, Come you, and let us go up to the mountain of the L-ord, to the house of the G-d of Jacob.." (Isaiah 2:3).
It is now clear from logic, scripture, and tradition that it is our duty to investigate into this of what we are capable of clearly grasping with our minds.
אַךְ לָדַעַת אִם אָנוּ חַיָּבִין לַחְקֹר עַל הַיִּחוּד בְּדֶרֶךְ הָעִיּוּן אִם לֹא, אֹמַר, כִּי כָל מִי שֶׁיּוּכַל לַחְקֹר עַל הָעִנְיָן הַזֶּה וְהַדּוֹמֶה לוֹ מִן הָעִנְיָנִים הַמֻּשְׂכָּלִים בְּדֶרֶךְ הַסְּבָרָא הַשִּׂכְלִית חַיָּב לַחְקֹר עָלָיו כְּפִי הַשָּׂגָתוֹ וְכֹחַ הַכָּרָתוֹ.
Regarding whether or not it is our duty to rationally investigate on the unity of G-d, I will say as follows: For anyone who is capable of investigating on this and other similar matters through rational inquiry - it is his duty to do so according to his intelligence and perception.
וּכְבָר הִקְדַּמְתִּי בִּתְחִלַּת הַסֵּפֶר הַזֶּה מִן הַדְּבָרִים הַמַּרְאִים חִיּוּב הָעִנְיָן מָה שֶׁיֵּשׁ בּוֹ דַּי וְהַמִּתְעַלֵּם מֵחֲקוֹר הֲרֵי זֶה מְגֻנֶּה וְנֶחְשָׁב מִן הַמְּקַצְּרִים בַּחָכְמָה וּבַמַּעֲשֶׂה וְהוּא דּוֹמֶה לְחוֹלֶה שֶׁהוּא בָּקִי בְּחָלְיוֹ וּבְדֶרֶךְ רְפוּאָתוֹ סָמַךְ עַל רוֹפֵא שֶׁמְּרַפֵּא אוֹתוֹ בְּכַמָּה מִינֵי רְפוּאוֹת וְהוּא מִתְעַצֵּל לְעַיֵּן בְּחָכְמָתוֹ וּסְבָרָתוֹ בִּרְפוּאוֹת הָרוֹפֵא לָדַעַת אִם הוּא מִתְעַסֵּק בְּעִנְיָנָיו עַל דֶּרֶךְ נְכוֹנָה אִם לֹא וְהָיָה יָכוֹל לַעֲמֹד עַל זֶה מִבְּלִי דָּבָר שֶׁיִּמְנָעֵהוּ. וּכְבָר חִיְּבַתְנוּ הַתּוֹרָה בָּזֶה כְּמוֹ שֶׁכָּתוּב (שם) וְיָדַעְתָּ הַיּוֹם וַהֲשֵׁבֹתָ אֶל לְבָבֶךָ וְגוֹ׳.
I have already written in the introduction to this book sufficient arguments which demonstrate the obligation of this matter. Anyone who neglects to investigate into it is blameworthy and is considered as belonging to the class of men who fall short in wisdom and conduct. He is like a sick man (a doctor) who is an expert on the nature of his disease and the correct healing method, but instead relies on another doctor to heal him who applies various healing methods, while he is lazy to inquire using his own wisdom and reasoning into the methods employed by the doctor, to see whether or not the doctor is dealing with him correctly or not, when he was easily able to do this without anything preventing him. The Torah has already obligated us on this, as written: "know therefore today, and lay it to your heart[, that the L-ord is G-d in heaven above and on the earth beneath; there is no other]" (Deut. 4:39).
וְהָרְאָיָה שֶׁהַהֲשָׁבָה אֶל הַלֵּב הוּא עִיּוּן הַשֵּׂכֶל הוּא מָה שֶׁאָמַר הַכָּתוּב (ישעיה מד) וְלֹא יָשִׁיב אֶל לִבּוֹ וְלֹא דַעַת וְלֹא תְבוּנָה וְגוֹ׳ וְאָמַר דָּוִד הַמֶּלֶךְ ע״ה (דה״א כח) וְאַתָּה שְׁלֹמֹה בְנִי דַּע אֶת אֱלֹהֵי אָבִיךָ וְעָבְדֵהוּ בְּלֵב שָׁלֵם וּבְנֶפֶשׁ חֲפֵצָה כִּי כָל לְבָבוֹת דּוֹרֵשׁ ה׳ וְגוֹ׳.
The proof that "lay it to your heart" refers to intellectual investigation, is from what the following verse says: "And none lays it to his heart, neither is there knowledge nor understanding" (Isaiah 44:19). So too David urged his son: "And you, Solomon my son, know you the G-d of your father, and serve him with a perfect heart and with a willing soul; for the L-ord searches all hearts" (I Chronicles 28:9).
וְאָמַר (תהלים ק) דְּעוּ כִּי ה׳ הוּא הָאֱלֹהִים וְגוֹ׳.
And David said: "Know you that the L-ord He is G-d" (Ps. 100:3).
וְאָמַר (שם צא) אֲשַׂגְּבֵהוּ כִּי יָדַע שְׁמִי וְאָמַר (ירמיהו ט׳:כ״ג) כִּי אִם בְּזֹאת יִתְהַלֵּל הַמִּתְהַלֵּל הַשְׂכֵּל וְיָדֹעַ אוֹתִי וְאָמְרוּ רַבּוֹתֵינוּ זַ״ל הֱוֵי שָׁקוּד לִלְמוֹד תּוֹרָה וְדַע מָה שֶׁתָּשִׁיב לָאֶפִּיקוֹרוֹס וְאָמְרָה תּוֹרָה (דברים ד) וּשְׁמַרְתֶּם וַעֲשִׂיתֶם כִּי הִיא חָכְמַתְכֶם וּבִינַתְכֶם לְעֵינֵי הָעַמִּים וְגוֹ׳.
And "Because he has set his love upon Me, therefore will I deliver him: I will set him on high, because he has known My Name" (Ps. 91:14), and "But let him that glories glory in this, that he understands and knows Me" (Yirmiya 9:23), and our Sages said: "be diligent in the study of Torah and know what to answer a heretic" (Avos 2:14), and the Torah says: "Keep therefore and do them; for this is your wisdom and your understanding in the sight of the nations.." (Deut. 4:6).
וא״א שֶׁיּוֹדוּ לָנוּ הָאֻמּוֹת בְּמַעֲלוֹת הַחָכְמָה וְהַבִּינָה עַד שֶׁיָּעִידוּ לָנוּ הָרְאָיוֹת וְהַמּוֹפְתִים וְעֵדֵי הַשֵּׂכֶל עַל אֲמִתַּת תּוֹרָתֵנוּ וְאֹמֶן אֱמוּנָתֵנוּ. וּכְבָר הִבְטִיחָנוּ יוֹצְרֵנוּ לְגַלּוֹת מָסָךְ הַסִּכְלוּת מֵעַל שִׂכְלָם וְשֶׁיֵּרָאֶה כְּבוֹדוֹ הַבָּהִיר לְאוֹת לָנוּ עַל אֲמִתַּת תּוֹרָתֵנוּ כַּאֲשֶׁר אָמַר (ישעיה ס) וְהָלְכוּ גּוֹיִם לְאֹרֶךְ וְגוֹ׳ וְאוֹמֵר (שם ב) וְהָלְכוּ עַמִּים רַבִּים וְאָמְרוּ לְכוּ וְנַעֲלֶה אֶל הַר ה׳ וְגוֹ׳.
And it is impossible for the nations to admit to our claims of superior wisdom and understanding unless there are proofs and evidences which can testify for us along with the testimony of the intellect on the truth of our Torah and our faith. And our Maker has already promised us that He will remove the veil of ignorance from their minds, and show His magnificent glory as a sign to us on the truth of our Torah when He said: "And the nations shall walk by your light" (Isaiah 60:3), and "And many peoples shall go and say, Come you, and let us go up to the mountain of the L-ord, to the house of the G-d of Jacob.." (Isaiah 2:3).
וּכְבָר הִתְבָּאֵר מִן הַשֵּׂכֶל וּמִן הַכָּתוּב וּמִן הַקַּבָּלָה שֶׁאָנוּ חַיָּבִין לְעַיֵּן בְּמָה שֶׁנּוּכַל לְהַשִּׂיג בֵּרוּרוֹ בְּדַעְתֵּנוּ:
It is now clear from logic, scripture, and tradition that it is our duty to investigate into this of what we are capable of clearly grasping with our minds.